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New Physiography 

for Beginners 


by 

Albert L. Arey/ C.E. 

Girls’ High School (Retired) 

^Frank L. Bryant/ B.S. 

Erasmus Hall High School 

William W. Clendenin/ M.S., M.A. 

Wadleigh High School 

J 

William T. Morrey, A.M. 

Bushwick High School 
in New York City 



D. C. HEATH AND COMPANY 

BOSTON NEW YORK CHICAGO LONDON 

ATLANTA SAN FRANCISCO DALLAS 

{ 






. 








Copyright, 1911, 1927 
By D. C. Heath & Co. 

2 L 7 



t 


JAN-3'28^ 


Printed in U. S. A. 



©Cl A101(809 5 






CONTENTS 


PART I 

THE EARTH AS A PLANET 

CHAPTER PAGE 

I. The Earth in Space. 3 

II. Latitude, Longitude, and Time. .... 25 

III. The Moon. 42 

IV. The Solar System. 50 

V. Map Projection. 68 

PART II 

THE AIR 

VI. Properties and Functions of the Air . 79 

VII. Temperature of the Air. 90 

VIII. Weight and Density of the Air .... 110 

IX. Movements of the Air.119 

X. Moisture of the Air.141 

XI. Light and Electricity of the Air . . . 156 

XII. Weather and Climate.171 

XIII. Climate of the United States ..... 199 

PART III 

THE SEA 

XIV. General Characteristics of the Sea . . 221 

XV. Movements of the Sea.235 

PART IV 

THE LAND 

XVI. The Mantle Rock .261 

XVII. The Bedrock.292 

iii 















iv CONTENTS 

CHAPTER PAGE 

XVIII. “Stories in Stones”. 342 

XIX. The Ground Water. 359 

XX. Rivers . 387 

XXI. Glaciers . .. 4 ^5 

XXII. Plains. 484 

XXIII. Plateaus and Mountains. 504 

XXIV. Volcanoes and Earthquakes ...... 540 

XXV. Shore Lines and Harbors. 568 










INTRODUCTION 


Since the publication of our previous text on physiography 
the authors have performed upon their classes many experi¬ 
ments on the pedagogy of the subject, some of which have 
resulted in new methods of presenting certain topics, others 
in new diagrams for certain explanations, and still others in 
new uses of the illustrations in the text. 

Several studies of illustrations are given in the text, e.g., 
the relative lengths of day and night, page 23; contour 
lines, page 75; local deposits of rivers, page 428; New York 
Harbor, page 588. 

The net result of these experiments is a broader view 
of the subject as a whole and a great increase in interest 
to the pupils, many of whom now spend more than the 
allotted time in preparation for their recitations, and some 
of whom major in the subject when they go to higher 
institutions. 

We believe that no illustration should be given room in a 
textbook unless it teaches something; and also that any 
teacher who follows the directions given in our pamphlet of 
Exercises and Review Questions will succeed in imparting 
the message of the pictures with a great saving of time and 
exertion. Furthermore, we believe that the pupil acquires 
clearer ideas of the physiographic features studied in this 
way than can be obtained in any other way except by field 
trips to the features themselves, which of course is not 
practical. 

At the close of the course the pupils will have considerable 
ability to read the history of a region from its picture. The 
teacher can test this ability by showing a picture that the 


VI 


INTRODUCTION 


pupils have not studied and by asking them to tell what has 
happened to the region. 

Forty per cent of the space in this book is occupied by 
illustrations, leaving but 360 pages of text to study.. The 
time spent on the illustrations need not be more than 10 per 
cent of that spent on the text and will materially help the 
pupil in his study of the text. 

Perhaps we can convey a better idea of our use of the 
illustrations by giving an outline of a lesson on Figure 202. 
When assigning the first lesson on stream erosion, ask the 
class to open their books to Figure 202 stating the following 
facts: 

1. In the foreground, just beyond the fence, is a creek that 
curves around a piece of flat land on the farther side of the 
stream. 

2. Beyond this flat land is a steep slope that rises 10 or 
12 feet above the flat to a gently sloping field. 

3. One day, after a heavy rain, the owner of the field 
found that a portion of his field had been washed away. 

Now ask each pupil to place the point of his pencil on the ' 
part washed out and to hold his book up so that you can see 
where it is. 

Next ask an individual pupil to describe the shape of the 
material washed out, and of the notch left in the field (the 
object is to get him to talk about it). 

Guide the talk by such questions as: 

What effect do such gullies have upon the value of the 
field? Why? 

Why does running water remove soil at one place and 
deposit it at another? 

Why is the washed out soil sometimes called a fan? 
A cone? 

Is there a gully shown? 

Be careful about the order of your questions. If your 
first question is, Where is the gully? some of the pupils may 
be looking for a little sea gull. 


INTRODUCTION 


vii 

Conclude the lesson by showing that the formation of the 
gully has illustrated erosion, transportation, and deposition, 
the work of all streams. This lesson may be used as the 
foundation for the work of streams. 

We have recently modified this procedure by using an 
enlargement of the picture in the book which can be placed 
where every one in the class can see it. This has two ad¬ 
vantages: it shows more detail than the small picture, and 
the pupil is guided by his own ideas rather than by the 
caption beneath the picture in the book. 

About twenty-five of the illustrations in the book have 
been enlarged and may be obtained from the authors. 

If teachers will cultivate their own enthusiasm for the 
subject, they will find it contagious and a material aid. 

The Authors 


DECEMBER, 1927 



























































4 


\ 





























. 































































PART I 


THE EARTH AS 


PLANET 





NEW PHYSIOGRAPHY 


CHAPTER I 

THE EARTH IN SPACE 

The earth is a ball nearly 25,000 miles around. It is com¬ 
posed of rock, with about three-fourths of its surface covered 
with oceanic waters having an average depth of only 2\ 
miles. Both rock and water are covered by a layer of mixed 
gases called the air. 

On its surface we are unconscious of any motion because 
of the steadiness and freedom from jar. Still we know that 
the rising and the setting of the sun are due to the turning 
of the earth on its axis. At night the star dome appears to 
revolve about the earth and gives us further evidence that 
we live on a ball that is turning in space uniformly and 
regularly. 

We also learn that, while the earth is whirling, it is rush¬ 
ing through space with inconceivable velocity. While the 
seconds pendulum of a clock makes one swing, the earth 
moves 18J miles, which is a thousand times the speed of the 
fastest express trains. 

In going this distance each second, the earth curves about 
one-ninth of an inch from a straight line. This slight rate of 
curvature, continued for a year, brings the earth around to 
the place of starting. 

The absolute uniformity of turning of the earth on its 
axis and the regularity of movement as a whole about the 
sun are of great service to mankind. The former affords a 
convenient means of measuring the length of the day, and 
the latter marks off the year. 

3 


4 


NEW PHYSIOGRAPHY 


The earth is only one member of a family of rotating balls. 
This family, together with other bodies, is controlled by the 
sun and constitutes a system. 

A conception of the earth in space, as a member of the 
solar system , and some knowledge of conditions on other 
worlds may give us clearer views of our real insignificance 
in space, time, and matter. 

With a velocity of 186,000 miles a second, light travels 
from the sun to the earth in about eight minutes, and from 
the nearest star in about four years. 

The orbit of the earth about the sun has a diameter less 
than one-half of the diameter of the nearest star. 

Condition of the Interior of the Earth. — The interior of 
the earth is believed to be solid throughout, although the 
temperature is undoubtedly above the melting point of 
materials on the earth’s surface. The melting point in¬ 
creases as the pressure, due to the weight of overlying 
rocks, increases; and it is believed that this pressure raises 
the melting point faster than temperature rises as the 
center is approached, so that the melting point is never 
reached. Because the earth as a whole has a greater den¬ 
sity than the material near the surface, the central portion 
is thought to be more dense than the so-called crust. 

Careful studies made of the variation in the position of 
the earth’s axis, the effects of tide-producing forces acting 
upon the earth, and the velocity of earthquake waves 
through the earth have led to the conclusion that the earth 
is more rigid than steel. 

Form, Size, and Weight. — Water and mud fly off from 
the fast rotating wheels of wagons and automobiles, when 
running on wet, muddy roads. This is due to the tendency 
of bodies to move in a straight line. When a body moves in 
a curved path, it appears to be pulling away from the axis 
of rotation. This pull is called centrifugal force. The 
greater the distance from the axis, the greater is the centrif¬ 
ugal force. 


THE EARTH IN SPACE 


5 


Because of the rotation of the earth, the excess of centrif¬ 
ugal force developed in equatorial regions causes the water 
of the ocean to bulge out there and to flatten in the polar 
regions. The resulting form is that of a slightly flattened 
sphere; an oblate spheroid. 

This does not necessarily indicate a one-time molten 



Fig. 1. — An Oblate Spheroid Representing the Form of the Ea.rth 

The flattening in the polar regions, N.P. and S.P., and the bulging in 
the equatorial regions, E and E', are necessarily in this figure much 
exaggerated. 

condition of the earth. The sea bulges at the equator and 
flattens at the poles, and the land wears down to sea level. 

The axis of rotation of the earth, or the polar diameter, is 
7899 miles, and the equatorial diameter is 7926 miles, the 
latter being 27 miles more than the former. The ends of 
the earth's axis are called poles. The average of the different 
diameters of the earth is nearly 8000 miles, and the circum¬ 
ference is about 25,000 miles. 

The surface area of the earth is nearly 197,000,000 square 












6 


NEW PHYSIOGRAPHY 


miles, of which 54,000,000 square miles are land. The 
earth is 5.6 times as heavy as a sphere of water the same size. 

Consequences of the Size and Shape of the Earth. — 
Because of modern inventions for the rapid transfer of 
thought, such as the telephone, telegraph, and radio, and 
means of transportation such as steamship, railroad, and 
flying machines, all parts of the world seem to be brought 
nearer together; and consequently the earth does not seem 



Fig. 2. — Method of Eratosthenes 

A, a vertical pillar at Syene, is 23|° north of the equator at E, a point 
on the Tropic of Cancer. B, a vertical pillar at Alexandria, is 7.2° or 
5000 stadia farther north. C is the center of the earth, and CA and CB 
radii. 

so large as it did before the use of these inventions. The 
size of the earth allows most effective use of these means of 
communication and travel. 

Because of its shape, all parts of the earth are actually 
brought nearer together than they would be if spread out 
on a flat surface, and therefore communication and travel 
are carried on with greater efficiency. 

Problem of Eratosthenes. — The first successful attempt to meas¬ 
ure the size of the earth was made about 200 b.c. by Eratosthenes, 
an astronomer and geographer of Alexandria, Egypt. He learned 





THE EARTH IN SPACE 


7 


that at Syene, the most southern city of ancient Egypt, the gnomon 
or vertical pillar cast no shadow at noon on June 21. At Alexandria, 
5000 stadia directly north of Syene, the sun’s noon ray on the same 
day made an angle of 7.2° with a vertical pillar. (See Figure 2.) 
Assuming the earth to be a sphere, this angle of 7.2° is equal to the 
angle formed at the center of the earth between radii to Alexandria 
and Syene. It follows that, as these two places are on the same 
meridian, an arc of 7.2° equals in length 5000 stadia, so that 360°, 
or the distance around the earth, equals 250,000 stadia. As the 
distance between Syene and Alexandria is about 500 miles, the cir¬ 



cumference of the earth would be 25,000 miles, which is not far from 
the truth. 

By selecting any two places on the same meridian, one may re¬ 
peat this experiment anywhere at any time. 

Evidences That the Earth Is Spherical. — 1. During 
every eclipse of the moon, that portion of the shadow of the 
earth cast on the moon always has a curved edge, appar¬ 
ently the arc of a circle. In Figure 3, at A the moon is enter¬ 
ing, and A' is leaving the earth’s shadow. The curved 
edge of the earth’s shadow shows that the form of the earth 
is spherical. The fact that the earth’s shadow on the moon 
is always a circular curve proves that the earth is spherical, 
because the only solid that can cast a circular shadow in 
all positions is a sphere. 

2. The circumnavigation of the earth both by ships and 



8 


NEW PHYSIOGRAPHY 


airplanes proves that the surface of the earth is curved and 
not flat as was formerly believed. 

3. As ships sail away, their hulls gradually disappear 
first; and as they come into port, the tops of smokestacks 
appear first. Before any part of the approaching ship is 
seen, the rising smoke from the smokestack often appears 
as coming out of the water. This shows that the water sur¬ 
face between the observer and the ship is actually curved 
upward and hides the distant ship. This curvature is found 
to be nearly the same in all directions on water surfaces, and 



consequently the earth may be said to be nearly a sphere 
in form. 

4. The weight of a body is about the same everywhere 
on the earth’s surface. This shows that the earth is ap¬ 
proximately globular in form. That the earth is not exactly a 
sphere is shown by the slight increase in weight in the higher 
latitudes. This is in part due to the flattening of the earth 
at the poles. The form of the earth is shown in Figure 1. 

5. As a result of the curved shape of the earth’s surface, 
places on different meridians have different times of day. 

If at A, Figure 4, the time of day is 9 o’clock, 15° farther 
west at the same instant it is 8 o’clock; at C, 15° farther 
west 7 o’clock; and so on around the world. If the earth’s 
surface were flat, all places would have the same time of day. 

6. On the shores of a calm lake, away from the tides and 
swells, the curvature of the earth may be measured by erect- 





THE EARTH IN SPACE 


9 


ing in a straight line three posts, A, B,C, at the same height 
above the surface of the water. (See Figure 5.) 


When one looks with a telescope from the top of post A to the top 
of post C, the top of post B will be above the line of sight. If the 


j** __ JJfjJC — - j 

f ML -. 

.——«*— r jlirva turc of the £ 

Lb_ I 

arth'3 surface - -- 


Fig. 5. — Post Method for Measuring the Curvature of the Earth 


distance from A to B is a mile, top of post B will be 8 inches above 
the line of sight, or the curvature of the water surface is 8 inches to 
the mile. In 2 miles the curvature is 8 inches X 2 squared, or 
32 inches; in 3 miles, 8 inches X 3 squared, or 72 inches — that is, 
the curvature for any distance is equal to 8 inches multiplied by the 
square of the number of miles. 

The Principal Movements of 
the Earth: A. Rotation. — The 

uniform spinning motion of the 
earth on its shortest diameter is 
called rotation. This shortest 
diameter is called its axis, the 
ends of which are known as poles. 

The line around the earth mid¬ 
way between the poles is the 
equator. 

As late as 1632, the time of 
Galileo, strong doubt existed as 
to the daily turning of the earth 
on its axis. The first experi¬ 
mental proof that the earth 
actually rotates was performed 
in 1851. Later experiments have proved this conclusively. 
(See Figure 6.) 

Foucault Pendulum Experiment. — In 1851 Foucault, a French 
physicist, devised a remarkable proof of the earth’s rotation by 



Fig. 6.—Foucault’s Experiment 













10 


NEW PHYSIOGRAPHY 


means of a pendulum. From the dome of the Pantheon in Paris he 
hung a heavy iron ball about a foot in diameter by a steel wire more 
than 200 feet long. The pendulum was set in motion, and the plane 
of vibration seemed to rotate slowly toward the right. It can be 
easily shown by a simple experiment that the plane of vibration of 
a pendulum remains fixed. The true interpretation must then be 
that the floor of the Pantheon was actually turning under the plane 
in which the pendulum was swinging. 

If the pendulum were suspended at the pole, the earth would 
turn around under it in 24 hours. The time required for the earth 
to shift entirely around under the plane of the vibrating pendulum 
increases as the latitude decreases. At the equator there would be 
no tendency for the earth to shift. 

The conclusion that the earth is rotating is confirmed by 
the fact that, with a telescope, we can see the rotation of 
the sun, the other planets, and some of the satellites. It 

has been found that 
one of the planets 
rotates in about 10J 
hours, another takes 
about the same time 
as the earth, and 
some others require 
months. 

Effects of the 
Rotation of the 
Earth. — We know 
now that one com¬ 
plete rotation 
occurs every day 
and that, in consequence of it, the sun, moon, and stars 
rise in the east, pass through the sky, and set in the 
west. 

As the earth receives its light from the sun, the side or 
half turned toward the sun is in light and has daytime. At 
the same time, the opposite side is in the shadow and has 



Fig. 7. — Rotation of the Eahth from above the 
North Pole 

Sunrise and sunset circles at the equinoxes. 
































THE EARTH IN SPACE 


11 



night as in Figure 7. The alternate occurrence of daylight 
and darkness is due to the rotation of the earth. The 
light and dark portions of the earth uniformly move west¬ 
ward because of the uniform eastward rotation. As we are 


Fig. 8. — Star Trails 

on the surface of the earth itself and rotating with it, we are 
unable actually to see it rotate. 

Star Trails. — Figure 8 is a sort of moving picture of the 
northern sky. If it were a snapshot, each star would make a 
point of light on the negative; but when the negative is 
exposed for a long time, each star makes a line of light that 
indicates its apparent change in position. 



12 


NEW PHYSIOGRAPHY 


This photograph was taken by pointing a camera toward 
the northern sky on a clear, moonless night and exposing a 
plate for about 9J hours. During the exposure the earth, 
carrying the camera with it about 15° an hour, caused the 
stars to trail on the plate. The trail near the center was 
made by the Pole Star itself, which is near, but not exactly 
at the north pole of the sky, the point which the earth's axis, 

if extended, would strike, and 
about which stars appear to 
rotate. 

The North Star was fo¬ 
cused at the center of the 
plate. The actual length of 
any trail will depend upon 
the distance from the North 
Star, but the length of any 
trail in degrees will be 15° 
for. each hour the plate was 
exposed. 

The stars appear to be mov¬ 
ing about the North Star in 
anticlockwise direction; that is, the stars above the North 
Star appear to move westward, those below eastward, 
those on the right or east rise, and those on the left or west 
sink. 

This apparent motion of the circumpolar stars is caused 
by the real motion of the earth, and the apparent anti¬ 
clockwise direction is due to the turning of the earth in the 
opposite direction. 

The angular rate of rotation of the earth is about 15° an 
hour as it turns through approximately 360° in 24 hours. 
The actual number of miles which a point on the earth’s 
surface moves due to rotation depends upon its distance 
from the equator. At the equator such a point moves 1000 
miles an hour, since the total distance traveled is 25,000 miles 
in 24 hours. 



Fig. 9. — The Angle between the 
Earth’s Axis and a Perpendicular 
to the Earth’s Orbit 




THE EARTH IN SPACE 


13 


In latitude 40° north or south, a body moves, because of 
rotation, a little less than 800 miles an hour. In 60° the rate 
of movement is one-half that at the equator. 

As the angular rate of rotation of the earth is everywhere 
uniform, it furnishes us with the most accurate timepiece 



Fig. 10. — Four Positions of the Earth in Its Orbit about the 
Sun Corresponding to the Four Seasons 


known. All clocks and watches are regulated by it. This 
clock of nature regulates our periods of rest and activity. 

The Principal Movements of the Earth: B. Revolution.— 
The motion of the earth around the sun is called revolution. 
The path of the earth about the sun is called its orbit , 







14 


NEW PHYSIOGRAPHY 


and the journey takes a few minutes less than 365 J days. 
This period of revolution determines the length of our year. 

The time it takes the other planets to go round the sun 
differs greatly. One planet goes around the sun four times 
while the earth goes once, whereas other planets require 
scores of earth years to make one complete revolution. 
The actual rate of the earth’s revolution around the sun is 
over 66,000 miles an hour, or nearly 66 times as fast as 



Fig. 10A. — The Earth on June 21 


the motion, due to rotation, of a particle on the earth’s 
equator. 

Causes for Our Regular Change of Season. — As the 

earth moves forward in its orbit around the sun, its axis 
remains tipped or inclined in the same direction and always 
the same amount. This inclination is 23J° from a perpen¬ 
dicular to the plane of the earth’s orbit. Because of the 
uniform inclination, the different positions of the earth’s 
axis are parallel. 

The causes of the regular changes of seasons may be 
stated as due to (1) inclination of earth’s axis, (2) its 
parallelism, (3) revolution. 



























THE EARTH IN SPACE 


15 


If the earth’s axis were perpendicular to the plane of the 
earth’s orbit, the sun would each day pass through the sky 
in the same path; all our days would be of the same length, 
and we should have no change of season. Because of the 
inclination of the earth’s axis, the sun passes through the 
sky at a much higher elevation in summer than in winter. 
The higher elevation of the sun causes long days and the 
lower elevation short days. Should the inclination of the 
earth’s axis be considerably increased above 23J° during a 



Fig. 105. — The Earth on December 21 


single revolution, the change of seasons would be more 
pronounced; that is, our summers would be warmer and 
our winters colder than they now are. 

The regular change of seasons depends upon the earth’s 
axis maintaining parallelism. Should the earth go around 
the sun with its axis inclined uniformly toward the sun, 
there would be no change of season. With the north pole 
tipped toward the sun during a revolution, perpetual summer 
in the northern and perpetual winter in the southern hemi¬ 
sphere would be the result. 

The length of the season depends upon the period of 






























16 


NEW PHYSIOGRAPHY 


revolution. Should the earth require a longer period of 
time to go around the sun, the seasons would be correspond¬ 
ingly longer. 

The change in distance of the earth from the sun during 
the year has little effect upon the change of seasons. As a 
matter of fact, our northern winter occurs when the earth 
is nearest to the sun, and northern summer when the sun is 
farthest away. 

The orbit of the earth has the form of an ellipse with the 


N 



Fig. IOC. — The Earth on September 23 
and March 21 


sun at the north focus. (See Figure 10.) The earth about 
January 1 is about 91,500,000 miles from the sun at a point 
in the orbit nearest to the sun known as 'perihelion. On 
July 1, the earth is 94,500,000 miles from the sun at a point 
the greatest distance away from the sun called aphelion. In 
Figure 10, the earth is shown in four positions as it makes 
its annual journey about the sun, a position for the astro¬ 
nomical beginning of each of the four seasons. The direction 
of the revolution is indicated by arrows along the earth’s 
orbit. On June 21 (Figure 10A), the pole, 90° north, because 
it is tipped toward the sun, is in the middle of the six-months 


































THE EARTH IN SPACE 


17 


period of sunlight. The pole, 90° south, is tipped away from 
the sun, because it is in the middle of the six-months 
period of darkness. Summer is beginning in the northern 
and winter in the southern hemisphere. 

The dayhght circle is tangent to the polar circles, and the 
sun’s vertical ray is at the Tropic of Cancer. 

On December 21 (Figure 105), the pole, 90° south, 
because it is tipped toward the sun, is in the middle of the 
six months of day. The pole, 90° north, because it is 


6 mos. at 90° 

4 V 2 mos. at 80° 

2 mos. at 70° 
18 h 30 m at 60° 
16 h 3 m at 50° 
14 h 51 m at 40° 
13 h 56 m at 30° 
13 h 13 m at 20° 
12 h 35 m at 10° 
12 h 0 m at 0 0 


Fig. 10D. — Duration of the Longest Day in 
Summer at Different Latitudes 

tipped away from the sun, is in the middle of the six 
months of darkness. Summer is beginning in the southern 
and winter in the northern hemisphere. 

The daylight circle is tangent to the polar circles, and the 
sun’s vertical ray is at the Tropic of Capricorn. 

On September 23 (Figure 10C), the sun rises at the south 
pole and sets at the north pole, and on March 21 it rises 
at the north pole and sets at the south pole. Spring is 
beginning in the northern and autumn in the southern hem¬ 
isphere. At these two dates, days and nights are equal in all 
latitudes. 











18 


NEW PHYSIOGRAPHY 


The daylight circle passes through the poles, and the sun 
is vertical at the equator. 

The following table shows the length of the longest day 
at points ten degrees apart in the northern hemisphere. 
The same periods give the length of the longest night in 
winter. (See Figure 10D.) 

Latitude 

0° (equator). 12 h 0 m 

10°. 12 h 35 m 

20°. 13 h 13 m 

30°. 13 h 56 m 

40°. 14 h 51 m 

50°. 16 h 9 m 

Cause of Unequal Days and Nights. — The causes stated 
for a change of season produce a shifting of the daily sky 
paths of the sun during the year, and the consequent varia¬ 
tion in length of daylight and darkness. 

On the equator of the earth the length of days and nights 
is always equal. The farther we go from the equator, 
either north or south, the greater the length of days and 
nights differs. The summer days are always long, and the 
winter days are always short at all places in the higher 
latitudes. 

The fundamental cause of this inequality of the length 
of days at different times of the year is the inclination of 
the earth’s axis. If the earth’s axis were inclined consider¬ 
ably more than 23|°, our summer days would be much longer 
and our winter days much shorter than they now are. 

On March 21 and September 23, the sun’s vertical ray 
being at the equator, the sun rises due east and sets due 
west; and days are everywhere equal in length to the nights. 
These two dates are called the equinoxes. On March 21 at 
the vernal equinox the sun is said “to cross the line,” and 
passes from the southern to the northern side of the equator; 
on September 23 at the autumnal equinox, the sun crosses 
the line again, at this time from the northern to the southern 
side of the equator. 


Latitude 

60°. 18 h 30 m 

66° 33' (Arctic Circle). 24 h 0 m 

70°. 2 months 

80°. 45 months 

90° (pole. 6 months 













THE EARTH IN SPACE 


19 


From the vernal to the autumnal equinox the sun rises 
north of east and sets north of west, and in the northern 
hemisphere the days are longer than the nights. In the 
southern hemisphere the nights are longer than the days. 
From the autumnal to the vernal equinox, the sun rises 
south of east and sets south of west, and in the northern 


Mi d'day Si 
'zenith n - 



Fig. 11. — Position op Sun’s Apparent Sky Paths at 
the Equator 

The paths are always perpendicular to the horizon, and days 
and nights are equal at all times of the year. 

hemisphere the days are shorter than the nights. In the 
southern hemisphere the days are longer than the nights. 

The northward journey of the sun ends on June 21, called 
the summer solstice. The southern journey ends on De¬ 
cember 21, called the winter solstice, when the sun again 
stands before starting northward. 

As shown in Figure 11, at the equator the daily sun paths 
are always perpendicular to the horizon, and all days are 
equal at all times of the year. The middle sun path marks 





20 


NEW PHYSIOGRAPHY 


the position of the sky equator and shows a vertical sun on 
the terrestrial equator only at the time of the equinoxes. 
The sun paths at the time of the solstices are 23^° from the 
sky equator. At the time of the summer solstice, the sun 
is vertical at the Tropic of Cancer; and at the winter sol- 


zenith 



Nadir 


Fig. 12. — Showing Position of Sun’s Apparent Daily 
Sky Paths at Latitude 41° N. 

The paths are tipped toward the south, showing our long 
days in summer and our short days in winter. The direction 
of sunrise and sunset at different times of the year may be 
read from the figure. 


stice vertical at the Tropic of Capricorn, each 23J° from the 
equator. 

In Figure 12, at latitude 41° north, the paths are tipped 
toward the south an angular distance equal to that of the 
latitude, this being the case in all latitudes from the equator 
to the north pole. South of the equator the same relation 
holds; only the sun paths are inclined north. Here the 
long days in summer and the short days in winter are shown 
by the relative length of sun paths above the horizon on 




THE EARTH IN SPACE 


21 


June 21 and December 21. The departure of sunrise from 
due east and sunset from due west is considerably more 
than at the equator. 

In Figure 13, the sun paths are inclined 66J° from a per¬ 
pendicular, that being the latitude of the Arctic Circle. 
Here the sun path for June 21 just touches the horizon at 



Fig. 13. — Showing the Position of the Sun’s Daily 
Sky Paths at the Arctic Circle, Latitude 663° N. 

On June 21 the sun remains above the horizon, and on 
December 21 it remains below the horizon for the entire 24 
hours. 

one point due north. From any position on the Arctic 
Circle on June 21, an observer would have an opportunity 
to see the sun due north on the horizon at midnight. 

At this latitude on December 21, the sun would appear 
on the horizon due south, at midday only. 

In the polar regions, however, where the periods of light 
and darkness may be many days or weeks, the habit of 
regularity of rest and activity is not generally formed. 
The long period of light in the summer season and the 
equally long period of darkness in the winter season make 








22 


NEW PHYSIOGRAPHY 


necessity and opportunity a period of work, and fatigue a 
time for rest. 

The earth’s rotation gives us a simple system of directions. 
East and west are roughly approximated by the rising and 
setting of the sun. The midday sun shows us which way is 
north or south, and the Pole Star at night marks north for 
those of us who live in the northern hemisphere. 


Zenith 



Fig. 14. — Showing Position of Sun's' Apparent Sky 
Paths at the North Pole 

Paths are nearly horizontal. The year is divided into 
two periods of sunlight and darkness. 


In Figure 14, the sun paths are parallel to the horizon, 
inclined 90° from a perpendicular, this position being that 
of the north pole, 90° north. At the time of the equinoxes 
the sun paths are at the horizon. The sun is said to rise 
at the vernal and to set at the autumnal equinox, causing a 
period of continuous sunlight for six months. During any 
period of 24 hours the sun apparently moves through the 
sky approximately at the same distance above the horizon. 
Should the altitude of the sun at intervals of 12 hours be 









THE EARTH IN SPACE 23 

found to be about the same, proof would be established 
that the observer was approximately at the north pole. 

As the sun sinks below the horizon about September 23, 
not to appear again until about March 21, a period of six 
months without direct sunlight obtains. 

This six-months period of night is not a period of total 
darkness. For nearly two months there is a gradual fading 
twilight as the sun continues to sink lower below the horizon. 
For about two months all light from the sun is cut off except 
that reflected by the moon. The dawn begins nearly two 
months before the sun finally appears again at the horizon 
on March 21. 

EXERCISE 

In Figure 1(M the full lines parallel to the equator are the parallels 
of latitude, 10° apart; and the twilight circle separates the illumi¬ 
nated from the dark hemisphere. 

Carefully examine this figure and write the answers to the fol¬ 
lowing questions: 

1. How does the portion of the equator that is illuminated com¬ 
pare with the portion in darkness? 

2. How would the length of the day at a point on the equator 
compare with the length of the night? 

3. Is the part of any parallel in sunlight longer than the part in 
darkness? If so, where? 

4. In what latitude is the difference greatest? 

5. Where is the portion of a parallel in darkness greater than 
the portion in sunlight? 

6. What part of Figure 10A is the summer hemisphere? The 
winter hemisphere? 

. 7. Study Figure 105, answering questions 3, 4, and 5. 

8. Study Figure 10(7, answering questions 3, 4, and 5. 

9. Summarize your answers, stating the relative lengths of day 
and night in the winter hemispheres; in the summer hemispheres. 

QUESTIONS 

1. Make a sketch of an oblate spheroid and draw in the axis and 
the equatorial and polar diameters. Properly letter the sketch and 
locate the poles. Where is the centrifugal force due to rotation the 
greatest? The least? 


24 


NEW PHYSIOGRAPHY 


2. Why are not the so-called evidences proofs that the earth is 
round? Which evidences are strongest? Which weakest? 

3. Try to picture in your mind, by using a globe, the actual path 
which a particle at the equator describes because of the combined 
motion of rotation and revolution. Make a free-hand sketch to 
show the motion and describe it. 

4. In about what latitude is the noon ray of the sun vertical on 
January 1? March 1? July 1? September 1? At these different 
dates, which is the longer here, the daytime or the night? In what 
latitude approximately are the northern and the southern limits of 
illumination at these dates? 

5. State whether at these different dates the sun rises north or 
south of east and sets north or south of west. 

6. Why are some star trails longer than others? 

7. When does the sun shine into north windows? 


CHAPTER II 


LATITUDE, LONGITUDE, AND TIME 

Latitude. — Our position on the earth in reference to the 
equator is called latitude. North latitude is north of the 
equator, and south latitude is south of the equator. The lati¬ 
tude of the equator is 0° and that of the poles 90°. # 

The equator is the circle extending around the earth mid¬ 
way between the poles. Circles parallel to the equator are 
called parallels. The planes of all parallels, as well as the 
plane of the equator, are at right angles to the earth’s axis. 
The distance expressed in degrees , minutes , and seconds, north 
or south of the equator, is called the latitude of a place. 

The axis of the earth extended northward marks the 
position of the north pole of the heavens. The elevation 
of the celestial or sky pole above the horizon equals the 
latitude of the observer. The angle between a vertical line 
and the plane of the earth s equator also equals the latitude 
of the observer. 

Because the equatorial bulge makes the curvature of the 
surface of the earth grow gradually less from the equator to¬ 
ward the poles, degrees of latitude increase slightly in length 
toward the poles. Less curvature of the earth’s surface in 
the higher latitudes means that the surface has the form of 
an arc of a larger circle. A degree, or 3 A 0 of the length of 
the circumference of a larger circle, is evidently longer than 
a degree of a smaller circle. A degree of latitude at any 
place is therefore 3 A 0 °f ^he circle whose curvature is that of 
the meridian at that place. (A meridian is a north and south 
line passing through the poles.) 

25 


26 


NEW PHYSIOGRAPHY 


The circle NESE' in Figure 15 represents a meridian section of 
the earth, NS being the axis and EE' the equator. HOH' is the 
plane of the horizon with the observer at 0. 

ON' extends north and is parallel to the axis NS. The point Z 
is the zenith directly over the observer. 

The angle OCE' is the latitude of the observer and equal to 
N'OH , the altitude of the north pole of the sky. 



Fig. 15 . — The Altitude of the North 
Sky Pole, Angle N'OH, Equals the 
Latitude of Observer O, Angle OCE' 


Proof. — Angle ZON', the ze¬ 
nith distance of the north pole 
of the sky, plus the angle HON' 
equals a right angle, or 90°, since 
ZO is the perpendicular to HH'. 

Angle NCO plus angle E'CO 
equals a right angle, or 90°, since 
the axis of the earth is perpendi¬ 
cular to the plane of the equator. 

Angles NCO and N'OZ are 
equal, since they are correspond¬ 
ing angles made by a line cross¬ 
ing two parallel lines. 

Therefore the complementary 
angle E'CO, the latitude of the ob¬ 
server, equals HON, the altitude 
of the north pole of the heavens. 


Latitude Determined by Night. — The latitude of an ob¬ 
server may be found on any clear night by means of the 
Pole Star (Polaris). The number of degrees of a heavenly 
body above the horizon is called its altitude. At the equator 
the North Star appears on the horizon, and its altitude is 
consequently 0. At 40° north of the equator, for instance, 
the North Star is 40° above the horizon (altitude 40°); 
and at the north pole of the earth, it is in the zenith (altitude 
90°). The altitude of the North Star in the northern hemi¬ 
sphere equals, therefore, the latitude of the place where 
the observation is made. This is not always absolutely 
correct, since Polaris describes daily a circle, 1J° from the 
north pole of the sky. 





LATITUDE, LONGITUDE, AND TIME 


27 


Latitude Determined by Day. — Another method of find¬ 
ing the latitude of a place is to measure the distance of the 
noon sun from the observer's zenith. At the time of the 
equinoxes the sun is on the sky equator, and the distance of 
the noon sun from the zenith equals the latitude of the place 
where the observation is made. 



Fig. 16. — Showing the Rotation of the Heavens about the North Star 


To find the latitude of a place at other times of the year 
by means of the zenith distance of the noon sun, certain 
corrections should be made. The Nautical Almanac gives 
the position of the noon sun in reference to the sky equator. 
This is called the sun's declination. In the northern hemi¬ 
sphere, if the sun is north of the sky equator, the zenith 
























28 


NEW PHYSIOGRAPHY 


distance of the noon sun will be that number of degrees less 
than the latitude. If the sun is south of the sky equator, 
the zenith distance of the noon sun will be just that number 
of degrees more than the latitude of the place. 

The zenith distance of the sun should be found just as it 
crosses the observer’s meridian, that is, when it is on a north 
and south line. 

Determining Latitude at Sea. — On shipboard the sextant 
is the instrument used to make observations to determine 
latitude. The sextant is a portable instrument used for 
measuring in degrees, minutes, and seconds the altitude of 
the sun above the visible sea horizon. 

At sunrise and sunset, the sun being on the horizon, the 
altitude is 0. At solar noon, the sun being directly over 
the meridian of the observer and at its highest point in the 
sky, the altitude is greatest for the day. To find the latitude 
of the ship, the sextant is used to find this meridian altitude, 
the sun’s declination is found in a Nautical Almanac, and the 
latitude is calculated as on land. 

Longitude. — The lines that pass from pole to pole on 
the earth’s surface are called meridians. Meridians are far¬ 
thest apart at the equator and converge toward each pole. 
The meridian that passes through Greenwich, England, is the 
prime meridian, and the meridian from which longitude from 
0 ° to 180° east and 180° west is reckoned. 

Longitude is the distance expressed in degrees, minutes, 
and seconds east or west, from the prime meridian. A degree 
of longitude at any place is -g-g-g- of the parallel of that 
place. 

The location of a place anywhere on the earth’s surface 
may be found by determining its latitude and longitude. A 
place in latitude 40° north and longitude 75° west is on the 
parallel 40° north of the equator, at a point where the merid¬ 
ian 75° west of Greenwich crosses it. 

How Longitude Is Determined. — To determine the 
longitude of a place, we find the number of hours, minutes, 


LATITUDE, LONGITUDE, AND TIME 


29 


and seconds, that the local time differs from that of the Green¬ 
wich observatory which is located on the prime meridian. 
One hour difference in time corresponds to 15° difference 
of longitude. One or more marine chronometers are carried 
on shipboard. These are merely large accurate watches 
mounted in boxes with swinging supports so that the rolling 
and pitching of the ship will not affect the keeping of very 
accurate time by the instruments. The chronometers are 



The shortest shadow, SN, falls at solar noon, the sun being on the meridian 
and highest in the sky. 

set to give the time of the meridian of Greenwich. The 
local time of the ship is determined by the sextant, used 
to observe the sun the moment it crosses the meridian. 

The time of day at certain land stations is now broad¬ 
casted by radio; and when it is received on shipboard, it is 
compared with local time, and the longitude of the ship is 
thus determined. 

Should the local time of the ship be one hour slower than 
Greenwich time, the ship is in longitude 15° west; and if one 
hour faster, 15° east. 






30 


NEW PHYSIOGRAPHY 


How to Find a North and South Line. — By the following 
methods, a north and south line may be located: 

(а) On any clear night the direction of Polaris, when it is directly 
above or below the sky pole (see Figure 16), is due north. This 
occurs twice in every twenty-four hours, when Polaris and Mizar, 
the star in the bend of the handle of the Big Dipper, are in a vertical 
line. 

(б) The direction of a magnetic needle when corrected for varia¬ 
tion, will enable one to locate a north and south line. 

(c) The direction of the shortest shadow cast on a horizontal 
plane by a vertical post is north and south. When the sun is at 
its highest point in the sky, shadows are shortest. This occurs at 
solar noon, which is approximately noon, local time. 

Navigation. — Finding the position of a ship on the open 
sea by observing the sun by day or the stars at night is called 
navigation by observation. 

Another method used when the sky is cloudy or in a fog 
is known as dead reckoning. Navigation by observation is 
used when possible. It is more accurate and is used as a 
check for error in dead reckoning. 

Dead Reckoning. — By keeping account of the course of a 
ship at sea and the distance traveled, the officers can plot 
the vessel’s position on a chart. This locates a ship on the 
ocean by dead reckoning. 

The course, or direction in which the ship is sailing, is 
indicated by the mariner’s compass and the speed by means 
of the log. 

“ Boxing the compass ” consists of reading the thirty-two 
points of direction on the compass card. 

The log consists of a rotator attached to a stout line which 
is dragged in the water from the stern of the boat. When it 
is thrown into the water, it is set rotating, the twist being 
carried by the log line to wheels of a recording dial. A pointer 
on the dial indicates the number of miles the ship actually 
travels each hour. 


LATITUDE, LONGITUDE, AND TIME 


31 


How Time Is Determined. — The rotation of the earth 
furnishes us with a measure of time. The day is a universal 
unit of time. It is the interval between two successive pas¬ 
sages across a given meridian of a given heavenly body. If 
the sun is the heavenly body taken for reference, the day is 
called a solar day; if the moon, a lunar day; and if a star, a 
sidereal day. 

The three kinds of days may be better understood from a study 
of Figure 18. E represents the earth in its orbit about the sun S, 



and E' is the position of the earth a day later. M represents the 
moon in its orbit about the earth, and W its position a day later. 
Far to the left of the diagram is a certain star so far away that lines 
drawn to it from any point on the earth’s orbit are practically 
parallel. The moon M l the sun S, and a star S' are on the meridian 

with the observer at 0. . 

The earth rotates as it moves forward in its orbit. The direction 
of the motion of revolution of both earth and moon and the direc¬ 
tion of the motion of the rotation of the earth, when seen from above 
the north pole, are counterclockwise, as indicated by arrows in 

fiS The real movement of the earth of approximately a degree a day 






32 


NEW PHYSIOGRAPHY 


in its path or orbit about the sun causes the sun to appear to move 
among the stars eastward about a degree a day. This has the effect 
of making the stars rise four minutes earlier and set four minutes 
earlier on successive nights. In a year’s time the stars come back 
to the same position in the sky at the same time of day, for four 
minutes each day of the 365 days of the year make about one 
whole day. 

The daily motion eastward of the moon in its orbit is about 13°. 
The earth must rotate 13° more than 360° to bring the moon again 
to the observer’s meridian; that is, the earth turns through 373° 
from 0 to O'", to complete one lunar day. 

Mean Solar Time. — Since the apparent motion of the 
sun is faster when nearer the earth and slower when farther 
away, the sun is a poor timekeeper. 

By taking the average length of all apparent solar days 
in a year, we obtain the definite length of our day. Our 

clocks and watches are regu¬ 
lated to keep this mean solar 
time. The apparent solar 
time read on the sun dial and 
the mean solar time read 
from our clocks agree only 
four times a year. This av¬ 
erage day is called the mean 
solar day, and may be con¬ 
sidered as beinq requlated by an 

Fig. 19. — Sun Dial . . 7 7 . 

imaginary sun that has a uni¬ 
form motion and consequently crosses the meridian at regular 
intervals. 

The attempt to construct clocks with compensating devices 
that would keep real solar time was made during the eight¬ 
eenth century. The variation in the sun’s apparent motion 
was so complex, however, that apparent time clocks were 
abandoned early in the nineteenth century. 

The sun dial consists of two essential parts, a style or 
gnomon and a dial. The style is placed parallel to the earth’s 






LATITUDE, LONGITUDE, AND TIME 


33 


axis and casts a shadow on the dial. The different hours of 
the day are marked on the dial; and the shadow of the style 
cast by the sun passing over it, as the sun moves through the 
sky, indicates the time of day. 

The style is usually a rod or edge of a thin plate of metal; 
and since it is parallel to the earth’s axis, it makes an angle 
with the horizontal dial plate equal to the latitude of the 
place where the sun dial is located. 

Equation of Time. — When the sun does not cross the 
meridian until after mean noon time, the sun is said to be 
slow; and when it crosses the meridian before mean noon, 
the sun is said to be fast. The amount that the real sun is 
ahead or behind the imaginary average sun is called the 
equation of time. 

This variation in solar time is published in the Nautical 
Almanac for each day of the year. Mean solar or clock time 
may be determined from a sun dial by reading the time indi¬ 
cated on the sun dial and then making correction by the use 
of the equation of time. 

For example, should the reading of the sun dial be 9 o’clock 
a.m. on a certain day and the equation of time gives the 
sun 10 minutes fast, the mean solar or clock time would 
be 8:50 a.m. 

The Civil Day. — Our ordinary day, called the civil day, 
begins at midnight and ends on the following midnight. 
Business is generally suspended at that time, and the change 
of date can be made then with the least confusion. The 
first 12 hours are called a.m. (ante-meridian), and the second 
period of 12 hours p.m. (post-meridian); 12 m. means noon 
or sun on the meridian. To find the exact time at which the 
sun is actually on the meridian, the table for the equation 
of time must be consulted or an observation must be 
made. 

For a person who travels around the earth, the number of 
times the sun crosses his meridian would be one less if going 
westward and one more if going eastward than it would be 


34 


NEW PHYSIOGRAPHY 


if he stayed at home. It is evident, then, that if the traveler 
does not add a day when going westward and drop a day 
when going eastward, upon his return his reckoning will 
differ one day from that at home. It has been agreed among 
mariners to make the change of date at the 180th meridian 
from Greenwich. 

To avoid confusion of dates on islands crossed by the me¬ 
ridian, an offset eastward a few degrees is made about New 
Zealand, and an offset westward is made about the Aleutian 
Islands. Another offset eastward is made to avoid passing 
across the extreme eastern part of Siberia. After passing 
through Bering Strait the date line returns to the 180th 
meridian. 

International Date Line. — The 180th meridian, together 
with the offsets mentioned, constitutes the international date 
line. (See Figure 20.) The date on the western side of this 
line being a day later than on the eastern side, ships in cross¬ 
ing it omit a day in their reckoning when going westward, 
and repeat a day when going eastward. For instance, if 
you were going by steamer from San Francisco to Japan, 
your time would be changed one full day when the steamer 
crossed this line; if you reached the line on Sunday morning, 
the day would then be called Monday. 

The Conventional Day. — By international consent, it has 
been decided that the day of any country at any moment is 
called the conventional day. The conventional day begins 
at the international date line, and moves westward 15° an 
hour with the sun. Parts of two different days are on the 
earth at the same time. The midnight line, which is just 
opposite the noon sun, marks the forward or westward 
boundary of each advancing day. 

Local Time. — The mean solar time of any place is called 
its local time. Places of different longitude differ in local time 
four minutes for each degree. In going around the earth at 
the equator, a distance of about 25,000 miles, the local time 
changes at the rate of one hour for a distance of about 1038 


Fig. 20. 


LATITUDE, LONGITUDE, AND TIME 


35 





















































































36 


NEW PHYSIOGRAPHY 


miles. In latitude 40°, a distance of about 801 miles makes 
a difference of one hour in local time, and in latitude 60°, 
519 miles. 

Standard Time Belts in the United States. — Because of 
the confusion that resulted from each place keeping its own 
local time, especially along railroads extending east and west, 
most railroad towns readily gave up their own and adopted 
the time in use by the railroad. The number of railroads 
increased until at certain centers there were many railroads 
entering the same city, each with a different local time in use. 
Much confusion arose from having different local times used 
in the same place. A definite system of keeping time in the 
United States was decided upon, and in 1883 the different 
railroad lines put it into operation. This system is called 
Standard Time, and may be defined as the time based upon a 
certain meridian that is adopted as the time meridian for a 
definite belt of country. Its advantage is that neighboring 
places keep the same time, instead of differing a few minutes 
or seconds according to their longitude. This is of especial 
importance in the operation of railroads and telegraphs, and 
in the transaction^ of any business concerned with con¬ 
tracts involving definite time limits. The standard time me¬ 
ridians of the United States, as adopted, are 75°, 90°, 105°, 
and 120° west from Greenwich. 

This system has been extended to the remote, possessions 
of the United States, and has spread over the greater portion 
of the world. 

Eastern Standard Time. — The mean solar time of the 
75 th meridian is used for places on both sides of that meridian 
in a belt approximately 15° wide. It is called Eastern 
Standard Time. This meridian runs through Philadelphia; 
local and standard time in that city are the same. The time 
within this belt is five hours slower than Greenwich time. 
The so-called time belts have very irregular eastern and 
western boundaries, depending upon the location of cities 
upon the railroads. 


Fig. 21. — Standard Time Belt 


LATITUDE, LONGITUDE, AND TIME 


37 



PACIFIC TIME MOUNTAIN TIME CENTRAL TIME EASTERN TIME 




































38 


NEW PHYSIOGRAPHY 


Central Standard Time. — The time of the next belt west¬ 
ward is the mean solar time of the 90th meridian, called 
Central Standard Time. It is one hour slower than eastern 
time. When it is noon, eastern time, at Washington, Balti¬ 
more, Philadelphia, New York, and Boston, it is 11 a.m., 
central time, at Chicago, Minneapolis, St. Louis, and New 
Orleans. 

Mountain Standard Time. — The next time belt westward 
uses the mean solar time of the 105th meridian, called Moun¬ 
tain Standard Time. Denver, Colorado, is on this meridian, 
so that clocks in that city indicate both mountain and mean 
solar time. 

Pacific Standard Time. — The time belt on the extreme 
west of the United States covers the states on or near the 
Pacific coast, and has the mean solar time of the 120th 
meridian, called Pacific Standard Time. Time in this belt 
is three hours slower than in the eastern belt, and eight 
hours slower than Greenwich time. In Alaska, standard 
time is nine hours slower than Greenwich time. On Jan¬ 
uary 1, 1919, the boundaries of the time belts were in 
most places moved and to some extent straightened. 

This change placed Florida, except in the extreme western 
part, in the eastern time belt, making time there one hour 
earlier than it had formerly been. 

Places located on the new boundary lines had a choice of 
times, and in most cases they decided upon the belt to the 
east of them with time one hour earlier. 

Time Signals. — The time service of the United States is 
under control of the government. By cooperation of the 
telegraph companies, time signals are sent out daily at noon, 
eastern time, from the Naval Observatory at Washington, 
D. C., to nearly every telegraph station in the country. 
These regulate automatically more than 30,000 clocks, and 
drop time balls in scores of different ports of the Atlantic, 
Pacific, Gulf of Mexico, and Great Lakes coasts. Time 
signals for the extreme western part of the United States are 


LATITUDE, LONGITUDE, AND TIME 


39 


distributed from Mare Island Navy Yard, in California. 
In recent years time signals have been sent by radio and at 
other hours besides noon. 

The Calendar. — The very early calendar worked out by 
the Romans was based largely on the motions of the moon. 
As the yearly number of revolutions of the moon varies, the 
seasons and festivals did not keep in place, and the Roman 
calendar fell into a state of great confusion. The year con¬ 
sisted of ten months, March being the first and December the 
tenth and last. January and February were added later. 
There were about 29 \ days in a lunar month; so the months 
were given 29 and 30 days alternately, beginning with 
January. The number of days in a week was probably 
based upon the number of planets then known, including 
the sun and moon. In 46 b.c., the Roman calendar was 
reformed by Julius Caesar, under the advice of Egyptian 
astronomers. 

The Julian Calendar. — The Julian calendar was planned 
without reference to the moon. It made three consecutive 
years of 365 days each, and the fourth of 366 days. The 
extra day was added to February, that month then having 
only 29 days, and the other months having alternately 30 
and 31 days. The length of the Julian year was 365.25 
days; and since the true year has 365.24 days, the Julian 
year was .01 of a day, or 11.2 minutes too long. 

This difference of 11.2 minutes between the length of the 
Julian year and the year now in use amounts to a little more 
than three days in 400 years. As a consequence, the date of 
the vernal equinox came continually earlier in the Julian 
year. In 1582 the vernal equinox occurred on March 11. 

The Gregorian Calendar. — In that year Pope Gregory 
XIII directed that ten days be stricken from the calendar, 
so that the spring equinox might occur on March 21. A 
further reform was introduced at this time in order to pre¬ 
vent a similar occurrence. The pope decreed that the cen- 
turial year should not be counted as a leap year except when 


40 


NEW PHYSIOGRAPHY 


divisible by 400. Thus 1800, 2100, and so forth, are not 
leap years; but 1600, 2000, and 2400 are leap years. 

The Gregorian calendar is now used in all civilized coun¬ 
tries except Greece and Russia, where the Julian calendar 
is still in force in spite of repeated efforts to abolish it. The 
fourteenth of every month here is the first of the month 
there. 

In England it was adopted in 1752. Dates of events oc¬ 
curring before the Gregorian calendar was adopted are 
termed Old Style (O. S.), and those after the adoption New 
Style (N. S.). 

In order to gratify the vanity of Augustus Caesar, the 
month now bearing his name, formerly called Sextilis, was 
given 31 days so as to have as many as July, formerly called 
Quintilis, which was named for Julius Caesar. A day was 
accordingly taken from February, leaving only 28 days for 
that month, and given to August. Because of a superstitious 
objection to having three consecutive months of 31 days 
each, September and November were reduced to 30 days, 
and October and December were given 31. 

QUESTIONS 

1. How may ships be located at sea? If city streets extend east 
and west and at right angles to avenues, how may places be located 
thereby? Compare the plan of locating a place in the city with 
that of locating the ship at sea. 

2. How may the following be determined in the southern hemi¬ 
sphere: (a) Latitude by night? (6) Latitude by day? (c) A north 
^nd south line? 

' 3. At what time of day is longitude usually determined? Why? 

4. What is the circumference of the earth at the 60th parallel, as 
compared with the circumference at the equator? 

5. Why is a solar day about four minutes longer than a sidereal 
day? Do solar days differ in length? Why? 

6. In laying out a north and south line by means of the noon 
sun, one should use the equation of time as a correction to clock 
time. Why? 

7. What are some of the practical advantages of having the civil 


LATITUDE, LONGITUDE, AND TIME 41 

day change at midnight? State any difference you may see between 
the civil day and the conventional day. 

8. How long has every day been on the earth before it reaches 
you? At what time by the clock at your place does a new day start 
on the earth? If Tuesday is just east of the international line, what 
day is just west of the line? Explain. 

9. By how much does the local time of your place differ from 
standard time? Why are the boundaries of the standard time belts 
so irregular? 

10. At what hour do the noon time signals from Washington 
reach Chicago? Denver? Explain. 

11. What advantages has the sun over the moon for calendar 
purposes? State the reason for the present rule for leap year. 

12. What advantage has radio over the telegraph in sending 
time signals? 


CHAPTER III 

THE MOON 

Distance, Area, and Size. — The moon’s average distance 
from the earth is about 240,000 miles. The actual distance 
during a single month varies about 30,000 miles, causing a 
corresponding variation in its apparent size. 

The diameter of the moon is 2163 miles; that is, about 
27 per cent of the diameter of the earth. 

The surfaces of the moon and earth are to each other as 
the squares of their diameters, or as 1 to 14. Their volumes 
are to each other as the cubes of their diameters, or as 1 
to 50. 

Real and Apparent Motion of the Moon. — The apparent 
motion of the moon and stars by night and of the sun by day 
is due to the earth’s rotation from west to east. There is a 
real eastward motion of the moon, as may be seen by noting 
the position of the moon among the stars from night to night. 

Since the moon makes one complete revolution about the 
earth in about 27| days, the eastward motion is about 13° 
a day; and as the sun also appears to move eastward among 
the stars about 1° a day, the eastward daily gain of the 
moon is about 12°. This causes the moon to rise about 
50 minutes later each day. 

Moon Has No Atmosphere or Water. — The moon has no 
appreciable atmosphere. Its absence is shown by the fact 
that, when the moon hides a star, the star disappears sud¬ 
denly and not gradually, as it would if its light passed 
through an atmosphere. There seem to be no effects of 
erosion on the moon, which also goes to show that there is 
no atmosphere there. If the moon ever had an atmosphere 
at any stage of its development, it has lost it. If water 

42 


THE MOON 


43 



Fig. 22. — Lunar Topography 


existed on the moon, it would evaporate during the long 
day there and form an atmosphere. 

Moonlight Surface Markings. — Moonlight is but re¬ 
flected sunlight. The surface markings on the moon are 





44 


NEW PHYSIOGRAPHY 


known to be due to a very uneven surface. The visible 
surface of the moon has an area about equal to that of South 
America, and nearly one-half of the area is covered with dark 
gray patches which were once supposed to be seas. The rest 
of the surface consists of mountains, so-called volcanoes, 
and craters and ringed valleys. Some mountain chains have 
peaks nearly four miles high. 

Same Face Is Always Toward the Earth. — Since the 
same side of the moon is always turned toward the earth, it 
follows that the period of rotation of the moon on its axis 
and its period of revolution about the earth are the same, 
about 271 da y s - Consequently we know nothing except by 
inference about the other side of the moon. The side of the 
moon that is toward the sun is always brightly illuminated, 
and the side turned away from the sun is in darkness. As 
the moon makes her way eastward around the earth, varying 
portions of the illuminated half are seen. This causes the 
moon’s phases. 

Phases of the Moon 

New Moon. — When the moon and the sun are on the 
same side of the earth, the dark side of the moon is turned 
toward the earth, and we have new moon. New moon, 
strictly speaking, occurs when none of the bright surface is 
visible. Popularly, the moon is said to be new when seen as 
a very thin crescent. A day or two later, when the moon 
has moved a little eastward of the sun, we may see in the 
early evening in the western sky a small portion of the illu¬ 
minated half in the form of a crescent , convex westward, or 
toward the sun, with the horns turned eastward, or away 
from the sun. 

First Quarter. — A week after new moon, half of the illu¬ 
minated hemisphere may be seen. The moon has now 
reached first quarter, and its shape is that of a half-circle. 
A line connecting it with the earth is at right angles to a line 
connecting the sun and the earth. As the moon passes be- 


THE MOON 


45 


yond the first quarter, the boundary line between the light 
and the dark area begins to be convex eastward, and the 
lighted portion continues to grow larger. 

Full Moon. — When the moon and the sun are on oppo¬ 
site sides of the earth, the whole lighted half of the moon is 
turned toward the earth, and we have full moon, about a 
week after the first quarter. The line dividing the light 

FIRST QUARTER 



Fig. 23. — Moon’s Phases 

The real illumination of the moon is shown in the inner eight positions in its orbit 
about the earth at E. The sun is at the right. The apparent illumination is shown in 
the corresponding outer position. 

and dark areas after full moon changes from the left side to 
the right side of the moon’s disk. 

Third Quarter. — The moon reaches the last or third 
quarter about a week after full moon. In this phase the half¬ 
circle is convex toward the left instead of convex toward the 
right, as seen in the first quarter. After third quarter, the 
moon being west of the sun, the crescent curves to the left 
or toward the sun, and the horns point to the right away 
from the sun. 






46 


NEW PHYSIOGRAPHY 


Waxing and Waning. — In its revolution from new to full 
moon, the visible illuminated area increases, and the moon 
is said to wax . From full to new the illuminated area de¬ 
creases and the moon is said to wane . 

Earthshine. — The dark portion of the moon is sometimes 
lighted by sunlight reflected from the earth, called earthshine. 
This occurs at the young and old crescent phases, and makes 
the entire disk of the moon visible. 



Fig. 24 . — Solar and Lunar Eclipses 


Eclipses 

Shadows. — All of the planets and their satellites are 
opaque bodies and cast long, cone-shaped shadows away 
from the sun. The length depends upon the size of the sphere 
and its distance from the sun. The average length of the 
earth’s shadow is about 866,000 miles and that of the moon 
232,000 miles. 

Cause of Eclipses. — The word eclipse as here used means 
a darkening of a heavenly body. This darkening may be 
real or apparent. The moon is eclipsed when it passes into 
the earth’s shadow; the sun is eclipsed when the moon 
passes between it and the earth. During a lunar eclipse, the 
moon is really darkened, light from the sun being cut off by 
the earth. During a solar eclipse, the sun is only apparently 







THE MOON 


47 


darkened; the moon cuts off light that would otherwise 
reach the earth. In reality it is the earth rather than the 
sun that is eclipsed. 

Total Lunar. — In the figure, the moon is passing through 
the earth’s shadow, BCD , and is totally eclipsed. The 
moon’s disk at this time is usually visible, however, because 
of sunlight bent into the earth’s shadow by our atmosphere. 
This gives the moon during a total eclipse a dull, copper- 
colored appearance. 

Partial Lunar. — When the moon passes slightly north or 
south of the center of the earth’s shadow, and only a part 
of the moon’s disk enters the shadow, a partial lunar eclipse 
occurs. The moon in its monthly revolution about the 
earth usually escapes the earth’s shadow entirely. 

Partial Solar. — Just outside the umbra of the moon’s 
shadow, an observer in the penumbra or partial shadow 
would see only a part of the sun’s disk, and would experience 
a partial solar eclipse. 



When the moon’s shadow is not long enough to reach to 
the earth and the moon passes centrally across the sun’s 
disk, leaving a ring of the sun’s disk exposed, the eclipse is 
said to be annular. The moon appears as a black spot 
covering the central portion of the sun’s disk, surrounded 
by a ring of light. 



48 


NEW PHYSIOGRAPHY 


Total Eclipse of the Sun. — The moon, being an opaque 
body, casts a shadow; and when between the sun and the 
earth, this shadow may be long enough to reach the earth 
and cause a total solar eclipse for those who are in the path 
of the shadow. 

For an observer so located, the moon appears to move 
over the sun and to cut off the light from the sun. 

The beginning of the eclipse is at the moment when the 
black body of the moon appears to cut a notch from the 
edge of the sun. The sun, as the moon gradually moves 
over it, appears as a diminishing crescent; and as the moon 
moves off, as an increasing crescent with the horns turned in 
the opposite direction. 

Near the beginning of totality the pearly white halo of the 
corona of the sun flashes out and is visible during totality, 
which is never more than eight minutes and usually much 
less. The time it takes the moon to cover the sun entirely is 
about an hour. 

Should any of the brighter planets happen to be near the 
sun during totality, they are generally recognized. Usually 
a few of the brightest stars also appear. 

Several streamers of light equal in length to the diameter 
of the sun and longer may be seen extending out from the 
corona. Near the sun red flames known as prominences 
appear. As the path of the moon’s shadow on the earth is 
always less than 170 miles wide and usually only a few 
thousand miles long, it is an event in a lifetime for an ob¬ 
server to see a total eclipse of the sun without making a 
journey to distant localities at the time this phenomenon 
occurs. 

In consequence of the eastward motion of the moon about 
the earth, the shadow moves eastward and with great veloc¬ 
ity. The earth by its rotation is carrying the observer in 
the same direction as the shadow moves. 

The total eclipse of the sun is of much scientific interest 
to astronomers, because it enables them by means of the 


THE MOON 


49 


spectroscope to discover new elements in the sun’s corona 
and because they can then obtain more accurate data con¬ 
cerning the moon’s motion and can study light and shadow 
phenomena. 

Number of Solar and Lunar Eclipses in a Year. — There 
are always at least two eclipses of the sun in a year, and 
there may be as many as four. The largest number of lunar 
eclipses in a year is three. As every eclipse of the moon is 
visible at one time from all points on one-half of the earth, 
and eclipses of the sun from a narrow area only, many more 
lunar than solar eclipses are visible at a given place. Rome 
during twelve centuries was privileged to see only three 
total solar eclipses and London two. 

QUESTIONS 

1. Compare the moon with the earth in respect to size and 
physical conditions. Where and when do we see the young crescent? 
The old crescent? How long is each usually visible? Why? 

2. During what phase of the moon do lunar eclipses occur? Solar 
eclipses? 

3. How many solar eclipses would occur each year if the orbits 
of the earth and moon were in the same plane? 

4. The time from full moon to full moon, called a lunar month, 
is 29^ days, and the actual time of a revolution of the moon about 
the earth is 27^ days. To what is this difference due? 

5. Why are total solar eclipses such a rare occurrence for any 
locality? 


CHAPTER IV 


THE SOLAR SYSTEM 

Solar System Defined. — The sun, together with the 
bodies revolving about it, is called the solar system. The 
members of the system are the sun, the planets and their 
satellites, the planetoids, some comets, and meteors. They 
may be briefly described as follows: 

1. The sun is near the center of the system, a very large, 
hot, self-luminous body giving heat and light to the other 
members. Its gravitative attraction controls their motions. 

2. The planets, eight in number, upon one of which we 
live, revolve about the sun in elliptical orbits, in different 
periods of time, and at different distances from the sun. 
Planets are distinguished from stars by their changing posi¬ 
tion among the stars and by their visible disk when seen 
through a telescope. Stars keep their relative position in 
the sky and through a telescope appear as points of light. 
Consult the following table: 


Planets 

Diameter 

in 

Miles 

Average Distance 
from Sun in 
Millions of Miles 

Period of 
Revolution 
in Years 

Number of 
Satellites 
or Moons 

Mercury. 

2,700 

36 

0.24 

0 

Venus. 

7,800 

67 

0.62 

0 

Earth. 

7,913 

93 

1.00 

1 

Mars. 

4,300 

141 

1.88 

2 

Jupiter. 

87,000 

483 

12.00 

9 

Saturn. 

72,000 

886 

29.00 

10 

Uranus. 

35,000 

1,782 

84.00 

4 

Neptune. 

32,000 

2,792 

165.00 

1 


3. All except two of the planets have satellites revolving 
about them. The satellites are very unevenly distributed 

50 


















THE SOLAR SYSTEM 


51 


among six of the planets, as seen in the foregoing table. 
Our moon is a satellite. 

4. The planetoids (planetlike bodies), about a thousand 
in number, are small bodies, as compared with any of the 
planets, and revolve about the sun between the orbits of the 
planets Mars and Jupiter. 

5. Comets are bodies that are temporarily visible, of 
large dimensions and small mass, unstable in form, usually 
with long tails and with uncertain orbits. Some comets re¬ 
volve about the sun in closed orbits, have fairly definite 
periods of revolution, and are consequently members of 
the solar system. Other comets with open orbits enter and 

Jupiter Saturn Uranus 


1 

Fig. 26. — Diagram of Orbits of the Planets Drawn to Scale 

then pass out of the solar system without becoming members 
of it. 

6. Meteors are comparatively small masses of stone or 
metal that enter the earth’s atmosphere from outside space. 
The light given out by them is due to their being heated by 
the friction and compression of the air. Meteors are many 
millions in number. Those which fall into the earth’s at¬ 
mosphere and become visible are familiarly called shooting 
stars. 

Size of the Solar System. — It will give us a better con¬ 
ception of the size of the orbits of the different planets if 
we draw to scale a map of the solar system. The orbits of 
the first four planets are so small compared with the orbits 
of the last four that it is difficult to find a suitable scale to 
represent the whole upon a single page of this book. The 
scale, one millimeter equivalent to 20,000,000 miles is used. 

Although the orbits of the planets are elliptical, they differ 
so little from circles that for this purpose the circle may be 
said to represent the planet’s orbit. 



Neptuna 








52 


NEW PHYSIOGRAPHY 


A picture of the solar system may be formed by means of 
Sir William Herschel’s apt illustration : 

Choose any well-leveled field. On it place a globe two feet in 
diameter. This will represent the sun. Mercury will be represented 
by a grain of mustard seed on the circumference of a circle 164 feet 
in diameter for its orbit; Venus, a pea, on a circle of 284 feet in 
diameter; the earth, also a pea, on a circle of 430 feet; Mars, a 
rather large pin’s head on a circle 654 feet; the asteroids, grains of 
sand in orbits 1000 to 2000 feet; Jupiter, a moderate-sized orange 
in a circle of nearly half a mile across; Saturn, a small orange on a 
circle of four-fifths of a mile; Uranus, a full-sized cherry or small 
plum upon the circumference of a circle more than a mile and a half; 
and finally Neptune, a good-sized plum, on a circle about two miles 
and a half in diameter. 

Space Outside the Solar System. — The known bodies 
occupying space outside of the orbit of Neptune are comets, 
meteoric swarms, large gaseous masses called nebulce, and 
stars. 

In literature the stars are often referred to as “ number¬ 
less ” and “ countless.” As a matter of fact, only about 
3000 stars can be seen without a telescope at any one time, 
and in the whole heavens there are fewer than 6000 stars 
that may be seen with the naked eye. With the telescope 
fainter stars are seen. The moderate-sized photographic 
telescope, with the modern sensitive plate, will show stars 
that are too faint to be seen with the largest telescopes. It 
has been estimated that the photographic plate has made 
record of about 100 million stars. Each of these stars shines 
by its own light and is in reality a sun. Many are more 
brilliant and larger than our own sun and may be centers of 
other systems. 

Our sun is a star and has a diameter 110 times that of the 
earth. The largest well-known star, Antares, has a diameter 
over 400 times that of the sun and 50,000 times that of the 
earth. 


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X 

















THE SOLAR SYSTEM 


53 


The Sun 

Diameter, Density, and Temperature of the Sun. — The 

sun is a huge sphere of incandescent gases and metallic 
vapors, with a diameter of 866,000 miles, and is 1.4 times as 
heavy as a sphere of water of the same size. Although but a 
small fraction of the total light and heat given out by the 
sun reaches the earth, yet nearly all life activities and most 
movements of air and water are due to this amount. 

The difference between conditions on the sun and those 
now on the earth is due largely to a difference in temperature. 

The Photosphere. — The visible surface of the sun is 
called the photosphere (light sphere). It is cloudlike in ap¬ 
pearance and gives forth most of the light and heat which 
the sun radiates. 

Sun Spots. — Dark spots of irregular outline, called sun 
spots, often many thousands of miles in diameter, mar at 
times the brightness of the photosphere. The sun spots are 
probably connected with the hidden circulation in the great 
body of the sun below the photosphere, and are dark only in 
comparison with it. Observers of sun spots soon found that 
the sun turns on its axis from west to east. The earth’s 
magnetism is disturbed during a period of unusual activity 
in the sun. A large number of sun spots appear, and a greater 
development of solar prominences occurs most frequently 
at these times. The period of maximum disturbance occurs 
on an average about every 11 years. 

As the sun rotates on its axis in about 26 days, no spot 
would remain continuously visible for more than 13 days, 
one-half of the period of the sun’s rotation. Some spots last, 
however, only a few days; others persist for months. They 
are now believed to be solar storms of great violence. 

Elements in the Sun. — By means of an instrument called 
the spectroscope, it is possible to tell some of the substances 
of which the sun is composed. About 40 elements, such as 
iron, carbon, hydrogen, nickel, silver, etc., familiar to us on 


54 


NEW PHYSIOGRAPHY 


the earth, are now recognized in a layer of gas overlying the 
photosphere. 

Chromosphere. — Outside this metallic layer is a deep 
envelope of gas, mostly hydrogen, called the chromosphere 
(color sphere). 

The chromosphere surrounds the sun and envelopes the 
photosphere. It is visible at the time of total eclipse. Portions 



Fig. 28. — Recent Photograph op the Sun Showing Spots 


The solar atmosphere is thousands of miles deep. The picture shows whirling storms, 
resembling the tornadoes of the earth, but of greater size, centering in the prominent 
sun spots. Photo by Mount Wilson Observatory. 

of it are thrown up in huge flamelike tongues. Astronomers 
have spent much time in studying it with the spectroscope. 

When the moon comes between the earth and the sun, the 
light from the photosphere is cut off, and the sun is said to be 
eclipsed. During the solar eclipse, the chromosphere can be 



THE SOLAR SYSTEM 


55 


seen as a brilliant scarlet ring. From its surface tongues of 
flame called prominences shoot out to altitudes of many 
thousands of miles. 

Corona. — The outermost portion of the sun is the corona 
(crown), a halo of pearly light extending out many thousands 
of miles, with streamers reaching out millions of miles. It is 
believed that the light of the corona is due to the reflection 
of light from dust particles, liquid globules, and small masses 
of gas. 

How the Sun’s Heat Is Maintained. — The theory that 
the sun’s heat is largely maintained by the gradual shrinkage 
of its volume is generally accepted. The fall of matter 
toward the center would continuously generate heat, as the 
blow of a hammer on a nail would heat both nail and hammer. 
Other sources of heat may add to the total amount that the 
sun sends out into space, such as that resulting from com¬ 
bustion, the falling of meteors, and radioactivity. 


The Planets as Individual Bodies 

Mercury. — So far as is known at present, Mercury is the 
smallest planet, the nearest to the sun, and the swiftest in 
its movements about the sun. It can be seen only in the 
direction of the sun during early twilight or late dawn. 
Mercury has a thin atmosphere, if any at all; it has surface 
markings of permanent streaks and a known rotation period 
equal in length to its year of 88 days. Since the periods of 
rotation and revolution are the same length, Mercury always 
turns the same side to the sun. This side is always heated 
and has perpetual daylight, while the side turned away from 
the sun is always cold and in darkness. 

Venus. — Venus shines in the sky with peculiar brightness. 
It has a diameter considerably more than double that of 
Mercury and only a little less than that of the earth. The 
period of rotation is now known to be 255 days, and equal 
to its period of revolution. Venus and Mercury are the only 


56 


NEW PHYSIOGRAPHY 


planets having equal periods of rotation and revolution. They 
pass between the earth and the sun, and consequently are 
the only planets that present all phases similar to those of our 
moon. The passages are called transits and occur at irregular 
and relatively long intervals of time. During these passages 
Venus and Mercury look like small, round, black spots pass¬ 
ing across the sun. 

The Earth. — Although we know that the earth is a planet 
moving about the sun like the other planets, the earth seems 
to us to be a center about which the other heavenly bodies 
move. The earth has the general form of the other planets, 
that of a spheroid. It is the third in distance from the sun 
and the largest of the four smaller planets whose orbits lie 
within those of the planetoids. The earth makes 366 rota¬ 
tions during one revolution. 

Mars. — Mars, though having only a little more than one- 
half the diameter of the earth, resembles it in more respects 
than any of the other planets. Its period of rotation is 24 
hours, 37 minutes, or a little more than our day. The incli¬ 
nation of its axis is about 24 degrees. Therefore, except for 
its greater distance from the sun, the days and change of 
seasons resemble those of the earth. 

Surface markings on Mars indicate to some astronomers 
snow fields and canals. There seems to be little doubt about 
the white polar caps that appear and disappear according 
to the season. It is not certain, however, that they are fields 
of snow. 

Although we have as yet no foundation from which to 
make any positive statement concerning the inhabitants of 
Mars, it may be maintained that, if any planet other than 
the earth is inhabited, it is probably Mars. Mars appears in 
our sky shining with a steady, pale red light. 

Jupiter. — Jupiter is the largest of all the planets, and, 
with the exception of Venus, often the brightest in the sky. 
Surface markings on Jupiter are described as parallel belts 
and spots. Because of the lack of the permanency of the 


THE SOLAR SYSTEM 


57 


markings, they are thought to be due to a deep atmosphere 
surrounding the planet. From observations of the spots, it 
has been found that Jupiter has a rotation period of about 10 
hours, which is the shortest known of any of the planets. 

The circumference of Jupiter is about 11 times the cir¬ 
cumference of the earth, with a rotation period less than half 
of that of the earth; the rate of rotation at the equator of 
Jupiter is about 30,000 miles an hour, nearly 30 times the 
rate of rotation at the equator of the earth. 

The outer four planets comprising the major group — 
Jupiter, Saturn, Uranus, and Neptune — are supposed to be 
of a higher temperature, of less density, and in not so ad¬ 
vanced a stage of development as the four planets Mercury, 
Venus, Earth, and Mars, comprising the minor group. 

Saturn. — Saturn is distinguished from all the other 
planets by three thin, flat, meteoric rings about 100 miles 
thick, easily visible through a small telescope, which surround 
it in the plane of its equator. The rings are together about 
40,000 miles wide, and the inner edge less than 6000 miles 
from the planet. At distances ranging from 100,000 miles to 
nearly 8,000,000 miles from Saturn, are 10 satellites, more 
than have yet been discovered belonging to any other planet 
in the solar system. 

The surface markings on Saturn are not seen nearly so 
well as those on Jupiter, because Saturn is nearly twice as 
far from us. There are bright and dark belts, and at times 
faint spots. Saturn rotates on its axis in about 10| hours. 
Because Saturn has a density of about three-quarters that 
of water, it is believed to be largely in a vaporous condition. 
It may be seen shining in the sky with a steady yellowish 
light, with about the same degree of brightness as the bright¬ 
est star. 

Uranus and Neptune. — Uranus was discovered in 1831, 
and Neptune in 1846. All the other planets were known 
to the ancients. Uranus is a very faint object in the sky, 
and Neptune is invisible to the naked eye. Neptune, the 


58 


NEW PHYSIOGRAPHY 


most remote of the planets now known, has the longest 
period of revolution, one year there being 165 earth years. 
It may be inferred that the physical condition of Uranus 
and Neptune is probably much the same as that of Jupiter 
and Saturn. The rotation period of Uranus, as indicated by 
surface markings, is between 10 and 12 hours. Since these 
planets are so far from the sun, they receive a small amount 
of heat per unit area compared with that received by the 
earth. 

Characteristics of Planets. — These characteristics may 
be briefly enumerated as follows: 

1. The planets move in the same direction about the sun 
from west to east. The sun rotates in this direction. The 
direction of movement as seen from above the north pole of 
the earth is opposite to the hands of a clock. 

2. The paths or orbits of all the planets are ellipses, with 
the sun at one of the foci. 

3. The other planets are nonluminous, like the earth; 
consequently the light that comes from them to us is reflected 
sunlight. 

4. Most of the planets are known to rotate in the same 
direction as the earth rotates, from west to east. 

The Satellites of the Solar System Compared. — Previous 
to 1610 the only satellite known was our moon. In that year 
Galileo first pointed his telescope to the sky and saw four 
large moons of Jupiter. Our moon is more than 2100 miles 
in diameter, but not so large as three of the eight moons of 
Jupiter and one of the 10 moons of Saturn. The largest 
satellite of Jupiter is 3558 miles in diameter, and considerably 
larger than the planet Mercury. The smallest satellites 
known are the two belonging to the planet Mars, both of 
which are probably less than 10 miles in diameter. One of 
the two is only 5800 miles distant from Mars, and makes a 
revolution in less than eight hours, one-third of the time it 
takes Mars to rotate. 

The earth’s satellite is about 240,000 miles distant from 


THE SOLAR SYSTEM 


59 


the earth and makes a complete revolution in about 27J 
days. The most distant satellite of Saturn takes considerably 
more than an earth year to make a revolution. The mass of 
our moon as compared with the mass of the earth is probably 
greater than the mass of any other single satellite, compared 
with the mass of its planet. Mercury and Venus have no 
satellites, Uranus has four, and Neptune one. 

Planetoids. — The planetoids, sometimes called asteroids, 
move about the sun just as the planets do. They are so small 



Fig 29. — Halley’s Comet 


that they are invisible to the naked eye. Not until the 
beginning of the nineteenth century were any of these bodies 
discovered. There was an early belief that an undiscovered 
planet revolved between the orbits of Mars and Jupiter. 
This, no doubt, led to the discovery of the first and largest 
planetoid, Ceres, 485 miles in diameter. There are several 
whose diameters are more than 100 miles, but the majority 




60 


NEW PHYSIOGRAPHY 


are much smaller, ranging down to about 10 miles in diam¬ 
eter. New ones are now being found every year by the 
method of photography. 

Comets. — Comets are in strong contrast with planets in 
appearance and physical condition. Most of them enter the 
solar system with orbits in the form of open curves, make one 
turn about the sun, and pass away, probably forever. 

Of the few comets that belong permanently to the solar 



Fig. 30. — Peary Meteorite 

In the American Museum of Natural History, New York City. 


system, all have definite periods of revolution about the sun, 
varying from 3.3 years (Encke’s Comet) to about 76 years 
(Halley’s Comet). Halley’s Comet last appeared during 
May, 1910. 

The typical comet is largely self-luminous and is composed 
of a head and a tail. In the center of the head is a bright, 





THE SOLAR SYSTEM 


61 


starlike nucleus surrounded by faintly luminous matter, 
called the coma. The tail acts like your shadow when you 
walk around a lamp. It always points away from the light. 
Some astronomers maintain that it is the pressure of sun¬ 
light that drives the gaseous particles from the nucleus and 
thus forms the comet’s tail. 

The head may have a diameter greater than that of the 
sun, with a nucleus as large as the earth and the tail equal 
in length to the distance of the earth from the sun. The 
amount of matter in a comet is very small, in most cases less 
than one-millionth of that of the earth. 

The orbits of the planets are slightly elliptical, and all are 
approximately in one plane; those of the comets are greatly 
elongated and lie in every possible position. With the un¬ 
aided eye it is a rare sight to see a comet. 

Halley’s Comet has been pursuing its fixed orbit about the 
sun since the dawn of history and undoubtedly long before. 
The accounts of many of its earlier appearances seem to 
indicate that it has been a conspicuous object. The last 
appearance, during May, 1910, was disappointing. This 
tends to show that the great comet has for ages been slowly 
disintegrating. 

Under the most favorable conditions the nucleus of 
Halley’s Comet was brighter than stars of the first magni¬ 
tude, the coma was a faint light, and the tail was a band of 
light about 8° wide at its widest place and 120° long. Stars 
were plainly visible through the comet’s tail. It is believed 
that the earth passed through the tail on May 18, 1910. 
At that time there were no unusual manifestations seen, 
such as the falling of an unusual number of meteors, a glow 
of the sky, or the appearance of deadly gases, all of which 
had been predicted. 

Meteors. — The earth in its path about the sun encoun¬ 
ters daily many millions of small bodies which enter its atmo¬ 
sphere from outside space. On a clear, moonless night, one 
may see several an hour. They often appear at altitudes of 


62 


NEW PHYSIOGRAPHY 


100 miles, move many miles a second, give out light and 
heat, and are usually consumed before they reach the surface 
of the earth. These bodies are called meteors. 

The appearance of an unusual number of meteors, usually 
in August and November, is known as a meteoric shower. 
Sometimes bodies weighing from a few pounds up to several 
tons fall to the earth’s surface unconsumed. Such bodies 
are known as meteorites. Some are composed of nearly pure 
iron, with a little nickel. Most meteorites are composed of 
stone, often with traces of iron in them. About thirty of the 
different elements found in the earth have been found in 
meteorites. 

Theories Concerning the Origin and 
Development of the Earth 

Nebular Hypothesis of Laplace. — Many hypotheses 
have been proposed, but the one that has exercised the 
greatest influence upon thinking people is the nebular hy¬ 
pothesis, as formulated by Laplace. This hypothesis main¬ 
tains : 

1. That the matter of the solar system was once a highly 
heated mass of gas called a nebula. 

2. That the form was a vast spheroid extending beyond 
the orbit of the farthest planet. 

3. That the nebula was in process of cooling, and the 
cooling caused shrinkage. An effect of shrinkage was to 
increase the rate of rotation, and this increased the equatorial 
bulge. 

4. That when the rotation increased to a certain speed, 
the centrifugal force at the equator of the spheroid equaled 
the attraction of the gravitation. Upon further cooling 
and contraction, the equatorial portion separated from the 
great rotating mass, forming a ring resembling the rings of 
Saturn. 

5. That as the cooling and contraction of the spheroid 
continued, additional rings were separated. The first ring 



THE SOLAR SYSTEM 63 

gave rise to the outermost planet, and the later ones to the 
other planets in turn. 

6. That the central body was the sun. 

7. That each ring parted at its weakest point, and the 


Fig. 31 . — Spiral Nebula 

matter was collected into a planet, which was hot and 
gaseous. 

8. That the cooling of the planet caused a contraction, 
which in turn increased the rate of rotation and consequently 
the amount of bulging. Some of the planets followed the 
example of the parent nebula and formed rings which became 
satellites. 



64 


NEW PHYSIOGRAPHY 


9. That as the cooling and shrinkage went on, the gases 
changed to a liquid and then to a solid state. In the case 
of the earth, the volume changed from a rotating mass ex¬ 
tending to the orbit of the moon to its present size. 

10. That the more volatile material of the earth remained 
in a gaseous state and formed our atmosphere, originally 
much deeper and of a higher temperature than now. As 
the atmosphere cooled, the water vapor condensed and 
formed clouds. As cooling continued, rain fell and the ocean 
formed. 

Planetesimal Hypothesis. — During the last few years 
the planetesimal hypothesis has been formulated. It may 
be stated as follows: 

1. The hypothesis starts with a cold nebula, spiral in form, 
which is the most common type now seen. 

2. The spiral nebula consists of a central portion or 
nucleus, which became our sun, with two arms starting from 
opposite sides and curved spirally. about the nucleus or 
center. 

3. A significant feature of the spiral nebulae is the pres¬ 
ence of numerous nebulous knots in the arms. These knots 
are the denser portions of the nebula, and the nuclei of future 
planets and satellites. 

4. The knots or nuclei are surrounded by a nebulous haze, 
which is composed not only of gaseous particles but also of 
innumerable solid or liquid particles. These particles re¬ 
volve about the center of the nebula like little planets. They 
are called planetesimals. The nuclei grow and become planets 
and satellites by the falling in of planetesimals. The earth 
and moon were two companion nuclei of unequal size. 

5. The earth developed from a knot in an arm of a spiral 
nebula by the capture of outside planet esimals. The in¬ 
creasing gravitational compression of the interior produced 
the internal heat of the earth. 

6. Gases were held in the solid planetesimals as they are 
held in meteorites that now fall to the earth. As the growing 


THE SOLAR SYSTEM 


65 


earth became heated by internal compression, the gases were 
given forth gradually, thus forming an atmosphere about 
the earth. Until the earth had attained a mass greater 
than that of the moon (Jt of the earth), its gravity was 
probably insufficient to enable it to hold the gases of an 
atmosphere such as we now know. The gases now issuing 
from volcanoes were occluded in the original planetesimals 
which formed the earth. 

7. When the earth had reached such size that water vapor 
was held in the atmosphere in sufficient quantity to reach 
the saturation point, the water vapor began to condense, 
and then the ocean began to form. 


The Two Hypotheses Contrasted 


Nebular Hypothesis 

1. Nebula, hot and large, formed 
rings around central mass or 
sun. 

2. Rings became planets. 

3. Smaller rings separated from 
planets and became satellites. 

4. Planets and satellites origi¬ 
nally hot and large, gradually 
cooling and growing smaller. 

5. Outermost planet, Neptune, 
formed first and others at 
later periods. 

6. Earth always had an atmos¬ 
phere. 


Planetesimal Hypothesis 

1. Nebula, cold, formed two 
arms around central mass or 
sun. 

2. Nuclei or knots became plan¬ 
ets and satellites. 

3. Smaller knots were captured 
by larger knots and became 
satellites. 

4. Planets and satellites origi¬ 
nally cold and small, gradually 
heating and growing larger. 

5. Planets and satellites form¬ 
ing at same time. 

6. Earth when small without an 
atmosphere. 


QUESTIONS 

1. What do we call the star about which the other members of 
the solar system revolve? 

2. Thousands of stars are greater in bulk and light-giving power 
than our sun. Why, then, does the sun seem so much larger and 
brighter than any of the other stars? 


66 


NEW PHYSIOGRAPHY 


3. What would happen to us if the sun were to be blotted out? 

4. The diameter of the sun is how many times that of the earth? 

5. Compare the diameter of the sun with the diameter of the 
orbit of the moon about the earth. 

6. The sun at its surface has an attractive force of 27.6 times 
that at the earth’s surface. How much, then, would a person 
weighing 125 pounds on the earth weigh on the sun? 

7. The circumference of the sun is 2,750,000 miles. Suppose a 
railroad train traveled at the rate of 60 miles an hour. How long 
would it take to go around the sun? 

8. Light travels 186,000 miles a second. How long does it take 
light from the sun to reach the earth? 

9. Dark spots are seen to move across the sun’s disk and to 
move always in the same direction. Just as straws show the rate 
and direction of the flow of a stream, what do these solar spots 
determine for us? 

10. Compare the periods of rotation of the sun, earth, and moon. 

11. How is it known that heavy incandescent gases and metallic 
vapors now form the sun? Name some of the elements in the sun. 

12. The sun attracts to itself as a common center all the bodies 
that go around it. Name these classes of bodies. What name is 
given to the system as a whole? 

13. What classes of bodies occupy space outside of this system? 

14. What distinguishes planets seen with the naked eye from 
stars? 

15. What two planets are termed inferior planets? What 
planets, then, are called superior planets? In this connection, what 
do inferior and superior mean? 

16. Explain how inferior planets, when examined through a 
telescope, pass through all the phases of the moon and how su¬ 
perior planets do not. 

17. Name the planets in order of size and also in order of dis¬ 
tance from the sun. 

18. Compare briefly the physical condition of each planet with 
those of the earth. What planets have you seen? 

19. How many times does Mercury go around the sun during 
one revolution of the earth? 

20. The period of rotation of Mercury is found to be equal to 
the period of revolution. How would this affect habitation on 
Mercury? 

21. With the exception of the moon, could any body in the solar 
system come as near the earth as Venus? Explain. 

22. Venus sometimes appears as a “morning star” and some- 


THE SOLAR SYSTEM 67 

times as an “evening star.” What does this mean, and under 
what conditions does it occur? 

23. Why is Venus often called the earth’s twin planetf 

24. The farther a planet is from the sun, the longer is the year 
of that planet. Compare the length of the year on Mars with that 
of the earth, Venus, and Mercury. 

25. The polar areas on Mars are denoted by white patches 
called snow caps, which increase and decrease in size at regular in¬ 
tervals; that is, when the north polar cap increases in size, the 
south polar cap decreases in size, and vice versa. What do these 
appearances suggest as to climate and seasons on Mars? 

26. What are the conditions on Mars favorable for types of life 
there of some kind or other? 

27. Photographic plates exposed to the sky for more than two 
hours exhibit, after being developed, a round dot for each star and 
a small elongated line for a planet. The number of these small 
elongated lines discovered from time to time now exceeds 1000. 
What are these bodies, more than 1000 in number, producing these 
elongated lines called? Why is the photograph of a star a round 
dot and that of a planet an elongated line? 

28. The giant world Jupiter, which is 1300 times as large as our 
earth, turns on its axis once in every 10 hours. What effect do 
the great size and rapid motion have on the shape of this planet 
as compared with the shape of the earth? 

29. “Saturn is the most beautiful and the most interesting of 
all our telescopic objects. The sight is unlike anything else in the 
universe.” For what reason was this statement applied to Saturn 
and not to any of the other planets? 

30. Why were Uranus and Neptune the only planets to be 
discovered? 

31. What planets have satellites? Account for some planets 
having more than others and some having none at all. 

32. What purposes do the satellites serve? 

33. What distinguishes a meteorite from a meteor? What is a 
meteor usually called? Why? 

34. How do comets differ in shape from planets and stars? 

35. Mention some of the peculiarities of comets. 

36. What bodies in the sky are lighted by the sun and what ones 
give light of their own? 

37. According to the nebular hypothesis, which planet was 
formed first? 

38. Give a reason why the outer planets are so much larger than 
the inner ones. 


CHAPTER V 


MAP PROJECTION 

Map making is one of the most important arts, and every 
great nation has a body of men engaged in surveying and map 
making. In the United States the General Land Office has 
mapped most of the country in order to allot and sell the 
public domain. The United States Geological Survey is mak¬ 
ing an accurate large-scale map to show geological and 
relief features, our navigable rivers, our lakes, and our 
coasts. On maps, then, we depend for the sale of our public 
lands, and the navigation of our rivers, lakes, and seas. 

A map is the representation of a portion of the earth's sur¬ 
face on a plane. The portion represented is indicated by its 
latitude and longitude. The scale of a map is the ratio be¬ 
tween the length of a line on the map and the actual dis¬ 
tance the line represents. The scale one mile to the inch is 
also a scale of -gweo because one mile equals 63,360 inches. 
On the United States Topographic Maps the scale most 
frequently used is ozroo, about one mile to the inch. The 
mapping of large areas with their curved surfaces and pole- 
ward converging meridians presents difficulties that are met 
by certain devices called projections. 

Map Drawing. — Pupils in drawing classes learn to draw 
a picture of a solid object like a block or a ball and to make 
it “stand out” by shading. 

Map making is a much more difficult operation. The per¬ 
fect map must indicate distances correctly without dis¬ 
torting shapes. Directions must be shown correctly, and 
the relief of the region must be shown. 

68 




MAP PROJECTION 


69 


Orthographic Projection. — One method of map making 
is called orthographic projection, because the points in the 
representation of the object upon the paper, called the 
plane of the picture, are determined by lines at right angles 
to the paper and 
therefore parallel to 
each other. Figure 
32 shows how this is 
done. 



Fig. 32 . — Orthographic Projection of a 
Rectangular Block 


ABCD is a horizon¬ 
tal surface like a floor 
and 1 2 3 4 is a block to 
be projected. The 
parallel dotted lines 
are the projection lines, 
and 1'2'3'4' is the pro¬ 
jection of the block on 
the paper. It is a view 

of the upper surface of the block as seen from a point far enough 
above, so that the lines of sight would be parallel to each other. 
This makes the projection the exact size of the upper surface of the 
block. 

In a similar manner a side view may be projected on a vertical 
surface like a wall. Let ABEF be such a surface. By using hori¬ 
zontal projection lines, we get the figure 3"4"5"6"which is the side 
view or vertical projection of the block. 


Orthographic Projections of the Earth. Figure 33 
shows the method of drawing polar and equatorial projec¬ 
tions of the earth by the orthographic method. It is used 
for representing hemispheres. 

A vertical line is drawn and at convenient distances apart, three 
lines, 1, 2, and 3, at right angles to it are drawn. Three circles of 
equal size are then drawn using the intersections, A, B, and C, of 
these lines as centers. 

The lower figure, marked A, is to be used for construction lines 























70 


NEW PHYSIOGRAPHY 


only. Assuming that the circle in A represents the equator and that 
the horizontal line marks the location of the prime meridian, we 
locate the points where meridians 30° apart cross the equator, num¬ 
bering them as in A. In the upper 
half of the circle in A plot angles 
of 23§° and 66|° with the equator. 

Equatorial Projection.— 

Diagram B in Figure 33 shows 
the earth as it would appear to an 
observer a long distance above 
the equator. From this loca¬ 
tion the equator would appear 
to be a straight line at right 
angles to the axis of the earth. 

We therefore mark the poles in 
their places on B and the hori¬ 
zontal line first drawn is marked 
equator. The 90th meridian falls 
on the vertical line first drawn and 
also appears as a straight line. 

We now project the other meri¬ 
dians in A upon the equator in B 
by dotted lines parallel to the first 
vertical line. Noting that the 
large circle in B is the prime meri¬ 
dian circle, we sketch the other 
four meridians, remembering that 
all meridians pass through the 
poles. 

To locate the tropics and polar 
circles, project the points where the angles of 23^° and 66^° intersect 
the circle in A upon the prime meridian circle in B, and draw 
horizontal lines through the four points on the prime meridian thus 
located. 



Fig. 33 . — Orthographic Projec¬ 
tions of the Earth 


Polar Projection. — Diagram B in Figure 33 shows the 
earth as seen from above the pole. The earth is supposed 





















MAP PROJECTION 


71 


to be rotated so that the axis points toward the observer. 
The horizontal line through the poles in C is therefore the 
prime meridian. 

Continue the projection lines of the angles of 23|° and 66J°, and 
mark their intersection with the prime meridian circle in C. Draw 
circles through these points using the pole as a center, to represent 
the Arctic Circle and the Tropic of Cancer. Now continue the 
30° and 60° projection lines from the equator of B to the equator 
of C, and draw the meridians as straight lines passing through the 
pole. 

In each of these projections we notice that distances- are fore¬ 
shortened near the edges. No scale of distances can apply to all 
parts of the map, but the projection is very useful because it shows 
the appearance of a hemisphere just as a globe does. 

Mercator's Projection. — Figure 34 is a Mercator’s pro¬ 
jection of the entire earth. 

We begin its construction with a horizontal line that represents 
the length of the circumference of the earth according to some chosen 
scale. 

If we divide this line into 18 equal parts, each part will represent 
20° of longitude and vertical lines through these points will repre¬ 
sent meridians, as in Figure 34. 

The meridians of the earth converge toward the poles, at 
a rate that makes a degree of longitude on the parallel of 
60° just half as many miles long as at the equator. In Mer¬ 
cator’s projection the degree of longitude is shown as the same 
length in all latitudes, and is therefore twice as long at the 60th 
parallel as it should he . 

Mercator corrected the distortion that would occur in the 
shape of land by making a degree of latitude twice as long 
at 60° as it is at the equator. This makes areas at 60° four 
times too large, but it preserves the true shape of the lands, 
and directions are correctly shown. For these reasons this 
projection is much used for mariners’ charts. 

Degrees of latitude are correctly represented at the equa- 


72 


NEW PHYSIOGRAPHY 






























































































































































































MAP PROJECTION 


73 


tor in this projection. At other points the length of the 
degree of latitude is increased in the exact proportion that a 
degree of longitude is decreased on the earth at the point in 
question. Figure 34 shows the gradually increasing length 
of the degree of latitude adopted by Mercator. 

Compare the size and shape of Greenland in Figure 34 
with its shape in Figure 75, in which the Mercator plan is 
not used. 

Mollweide Projection. — In this projection the equator 
and a meridian are laid off their true relative lengths at 
right angles to each other at their midpoints. The true rel- 



Fig. 35 . — Mollweide Projection of the Meridians and Parallels 


ative distances between parallels are laid off along the 
meridians and the true relative distances between merid¬ 
ians along the equator. Ellipses are then drawn passing 
through the poles and the proper points on the equator 
to represent the meridians. The parallels are then drawn 
parallel to the equator. This projection is being used more 
and more. It is pleasing to the eye and has the great ad¬ 
vantage of showing the entire earth. There is a slight exag¬ 
geration in polar regions and quite a distortion of shape. 

Globes. — Globes represent the whole earth; they show 
the relative sizes of the land masses and their relative posi¬ 
tions correctly, and directions are correctly indicated on 
them. It is not possible to make them large enough to show 













































74 NEW PHYSIOGRAPHY 

the amount of detail necessary for many purposes, although 
sections of globes have been made that show limited areas 
with sufficient detail for most purposes. 

Relief. — The elevations and depressions of the earth’s 
surface constitute what is technically known as the relief 

of the earth. It is best repre¬ 
sented by models in which a 
greater scale is used for the 
elevations and depressions 
than for horizontal distances. 

Models are expensive, 
cumbersome to handle, and 
limited to the representation 
of small areas. 

Figure 36 shows one way 
of representing the relief of a 
region on a flat surface. 
Lines called hachures are 
drawn to indicate the paths 
that water would follow in 
flowing down the slopes. 
Sometimes steep slopes are 
indicated by short wide 
lines and gentle slopes by narrow lines farther apart. 

Hachure maps give us a general idea of the relief of a re¬ 
gion, but we cannot tell whether the ridges are a hundred 
feet high or thousands of feet high. This method was for¬ 
merly used by the United States Coast Survey, but has been 
abandoned in favor of contour lines, a method described 
hereafter. The change resulted in a large decrease in the 
cost of drawing and engraving maps, but the information 
is more accurate. 

Contours. — Lines passing through points having the same 
elevation above sea levels are called contours. 

The difference in elevation represented by two adjacent con¬ 
tour lines is called the contour interval. 



of the Austrian Alps 


MAP PROJECTION 


75 


Contour maps give definite information as to the elevation of 
all points on the maps that are touched by contour lines and 
enable us to estimate the elevation of points between the lines. 



EXERCISE 

Figure 37 is a map of Grass Lake and Red Creek. The figures 
indicate the elevation above the surface of the lake of the point 
indicated by the period nearest to the figure. 

The zero contour is the shore line. Note that the line passes 
through the periods rather than the figures. 

Let the student trace the map and plot the 10-foot contour on the 
tracing. 










76 


NEW PHYSIOGRAPHY 


(Note. The 10-foot contour line separates elevations less than 
10 feet from those greater than 10 feet. It never passes between 
two numbers, both of which are greater than 10 feet, nor between 
two that are less than 10 feet, because the surveyor always locates 
every change in slope. 

If two adjacent numbers should be 10 and 40, you must plan to 
have both the 20-foot and the 30-foot lines pass between them. 

Plot all contour lines using a 10-foot interval, and answer the 
following questions.) 

1. In what part of the map is the land flattest? How is this 
shown? 

2. In what part is it steepest? How shown? 

3. If you were to walk along Red Creek from the north, should 
you be going downhill or uphill? How do you know? 

4. What changes of level would be experienced by a person 
walking from the top of East Hill to the top of West Hill? 

5. When a contour line forms an angle that bends away from a 
shore line, does it indicate a hill or a valley? 

6 If you were on the top of East Hill, could you walk in any 
direction without changing level? What land form is indicated by 
closed contours? 

7. How does East Hill differ from West Hill as to elevation? 
As to area of its summit? 


QUESTIONS 

1. Why are methods of map projection necessary? 

2. What advantage has a globe over an orthographic projection 

of the earth? . 

3. What advantage has an orthographic projection over a glober 

4. State the advantages of the Mercator projection. 

5. Locate two points not within sight of each other, on Fig¬ 
ure 37. 


PART II 


THE AIR 

/ 
































































































■ 



































































. 






























































CHAPTER VI 


PROPERTIES AND FUNCTIONS OF THE AIR 

Introduction. — No part of his environment is of more 
immediate concern to man than the air he breathes. If it 
is pure, he is strong; vitiate it, and he sickens. Withdraw 
it but for a single hour, and he dies. 

No other part of his environment has had so great an in¬ 
fluence in helping or retarding him in his struggle for exist' 
ence or in his effort to improve his condition. How he 
dresses, what he produces, and what he eats are matters 
chiefly of weather and climate. Too great heat and too 
great cold are alike prohibitive of higher aspirations for 
better things. 

The savage blacks of equatorial Africa and the Eskimo of 
the frozen north are both low in the scale of civilization; the 
first because the enervating climate destroys ambition; the 
second because providing for mere physical needs exhausts 
his energies, leaving no opportunity for cultivation of the 
higher qualities. Both must adapt themselves to their 
climatic environment; neither can change it. 

Definition. — The earth’s atmosphere, or air, is the outer 
gaseous part of the earth. It envelops the solid and liquid 
parts, extending to a height of probably more than two 
hundred miles, and fills all mines, caves, and underground 
passages. As ground air it penetrates all soils and by the 
movements of the water it is carried to the greatest depths 
of rivers, lakes, and seas. 

Depth of the Air. — The most reliable estimate of the 
depth of the air is that based upon the height at which 
meteors are seen. These dark bodies in their revolution 
about the sun are sometimes drawn from their orbits by 

79 


80 


NEW PHYSIOGRAPHY 


the attraction of the earth. When this occurs and they 
enter the earth’s atmosphere, the compression of the air in 
front of them, together with the friction of the air as they 
fly with great velocity, causes them to become luminous and 
therefore visible. 

Since meteors have been seen at heights of more than 200 
miles, we can say with assurance that the air is much more 
than 200 miles deep, as the meteors must travel far after 
entering the rare upper air before they become white hot. 

Properties. — Pure air is an invisible gas, colorless, odor¬ 
less, and tasteless; very compressible and perfectly elastic. 
It is very mobile and, like all matter, has weight. Though 
under ordinary conditions gaseous, it may easily be made to 
assume the liquid state. 

The compressibility and elasticity of the air make possible 
its substitution for steam in driving machines. This use is 
particularly important in deep mines where the long dis¬ 
tances it must be carried results in condensation of the 
steam. 

All matter persists at rest or in uniform motion until 
some force changes it, because of inertia. The inertia of the 
air at rest causes resistance to motion through it, retarding 
the speed of the runner, the automobile, the express train, 
and the airplane. On the other hand, it makes possible the 
flight of birds and man in heavier-than-air machines. 

The inertia of air in motion causes pressure on objects not 
moving with it, which varies as the square of its velocity. 
This is put to practical use in driving ships and turning 
windmills. As our fuel supplies fail, we must, by harness¬ 
ing the winds, avail ourselves more and more of this source 
of unlimited power. 

The buoyancy of the air aids in the distribution of seeds, 
the sailing of balloons, and the* flight of airships. When 
the bag of the balloon or cells of the airship are filled with 
a gas lighter than air, usually hydrogen or helium, the buoy¬ 
ancy of the air causes the machine to rise, or rather, to be 


PROPERTIES AND FUNCTIONS OF THE AIR 81 


pushed up. It will continue to rise until it reaches air of 
such density that it displaces just its own weight of air. 
Balloons made of thin rubber which permits the gas to ex¬ 
pand as the balloon rises, thereby displacing more air and 
enabling the balloon to rise higher, have been sent to heights 
of almost 20 miles. The greatest height to which a manned 
balloon has risen is about seven miles. 

Since balloons fly with the wind, they cannot be steered. 
By combining the two properties of buoyancy and inertia of 
the air at rest, the airship has been evolved. Like the bal¬ 
loon, the airship is buoyed up; like the airplane, it is driven 
by motors and can therefore be steered. 

Composition. — The lower air is essentially a mechanical 
mixture of nitrogen, oxygen, carbon dioxide, and argon. 
Water vapor, water particles, and dust are usually present in 
it. The relative amounts of the first four are nearly con¬ 
stant, while the last three are extremely variable. 

Nitrogen and oxygen bear to each other about the ratio 
of 78 to 21 by volume, and 76 to 23 by weight. Carbon 
dioxide constitutes about .03 per cent of the air, varying 
slightly with locality and season. Of argon little is known, 
its existence not having been discovered until recent years. 
Argon constitutes about one per cent of the air. It was 
formerly included with nitrogen. 

Essential Composition of the Air 

Nitrogen.78.00% 

Oxygen.21.00% 

Argon. 1.00% 

Carbon Dioxide.03 % 

Distribution of Components. — In obedience to the prin¬ 
ciple of diffusion (ready and spontaneous mixing), the gases 
of the air make a fairly uniform mixture. Local conditions 
may temporarily disturb this adjustment, but on the whole 
the air of one region of the earth is like that of any other. 

Carbon dioxide, since it is one of the products of volcanic 






82 


NEW PHYSIOGRAPHY 


action, is most abundant in regions of active volcanoes. 
Being likewise a product of decomposition and combustion, 
it is more abundant in cities, especially in manufacturing 
cities, than in the country; and more abundant in winter 
than in summer. The use by growing plants of carbon 
dioxide tends to decrease still further its summer percentage. 

Water vapor, though always present in the air, is not an 
essential component. It is one of the most variable con¬ 
stituents of the air and is in general more abundant over the 
sea than over the land, in low than in high altitudes, and in 
summer than in winter. 

Water and ice particles in the air, known as cloud, fog, 
mist, rain, snow, hail, and sleet, are limited to the lower 
air, reaching an altitude of only a few miles. 

\ Dust in the air is of two kinds, organic and inorganic. 
Organic dust includes microscopic animals and plants, 
pollen, fibers of wood and cloth, and the soot of smoke. In¬ 
organic dust consists chiefly of powdered minerals and rocks 
derived from the land and caught up by the winds. Dust is 
more abundant over the land than over the sea and is confined 
to the lower air. It is more abundant in cities than in the 
country and in dry than in rainy weather, the dust particles 
being carried down by the falling rain drops. 

Mountain health resorts are sought partly because of the 
greater dryness of the air and partly because of its freedom 
from dust and the disease germs that constitute part of 
the organic dust of the air at lower altitudes. 

Ozone, sometimes considered a constituent of the air, is 
really oxygen under peculiar conditions. By the passing of 
an electric spark through air, the oxygen is in part changed 
to ozone, which, however, changes back to the more stable 
condition of oxygen. 

The invigorating quality of the air after a thunderstorm 
is thought to be due, in part at least, to the ozone produced 
by the passage of lightning flashes through it. The per¬ 
centage of ozone increases with the altitude. 


PROPERTIES AND FUNCTIONS OF THE AIR 83 

The composition of the air changes materially as we rise 
ir it. The dust, water particles, and even water vapor 
r ipidly decrease above the height of a few thousand feet, 
and the percentage of oxygen increases slightly. Carbon 
dioxide decreases in percentage, and it is probable that at 
great heights there is free hydrogen in the air. 

Function of the Air. — Although the most important uses 
of the air are those of its individual components, yet the air 
as a whole has important functions. By virtue of air, flight 
of birds and man is made possible, and sounds are trans¬ 
mitted. By air in motion ships and windmills are driven, 
life-giving and disease-producing germs are carried, and the 
seeds of many plants are fertilized and distributed. Rain 
is distributed over the lands, and waves and ocean currents 
are produced. Tornadoes and hurricanes, with all their de¬ 
structive power, are but air in violent motion. 

As a carrier of waste from higher to lower levels, thereby 
wearing down the lands, and in the accumulation of sand 
dunes and loess deposits, the air is an important geological 
agent. Its presence in the mantle rock promotes disintegra¬ 
tion of the minerals and the production of soil. 

One of the chief purposes in cultivating crops is to increase 
the amount of ground air. When the surface is packed by- 
rains and remains unbroken by cultivation, air penetrates 
the soil with difficulty, and growing crops languish. 

Function of Oxygen* —^gjie oxygen of the air is the sup¬ 
porter *f combustion. its chemical union with other 

elements heat is evolved-' This process, called oxidation, 
may be slow, as in the resting of metals, in which case the 
heat radiates as rapidly as produced, and there is no per¬ 
ceptible increase of temperature; or it may be rapid, as in 
the burning of wood, coal, or oil, resulting in an increased 
temperature and often in the production of light. By com¬ 
bination with carbon in the blood of animals, oxygen sup¬ 
plies the heat necessary to animal life. 

The readiness with which oxygen unites with most other 


84 


NEW PHYSIOGRAPHY 


chemical elements makes it active in promoting the dis¬ 
integration of rocks and minerals. It is an important agt. nt 
in the decomposition of dead animal and vegetable matte-, 
thus serving as a purifier of the air. In the form of ozone 
its activity is increased. 

Oxygen is more soluble in water than are the other con¬ 
stituents of the air. The percentage of oxygen in air en¬ 
meshed in water is therefore greater than in ordinary air, 
its ratio to nitrogen by volume being 34 to 66 instead of the 
ordinary ratio of 21 to 78. It is this enmeshed air, obtained 
at the surface and carried by currents to the greatest depths 
of all lakes and seas, that makes life possible, even in the 
profoundest deeps. 

Function of Carbon Dioxide. — The carbon dioxide of 
the air, though of no direct use to animals, is essential to the 
life and growth of plants. Through the action of sunshine 
and the chlorophyl, or the green matter of the plant, carbon 
dioxide, absorbed mainly through the leaves of the plant, 
is broken up, the carbon retained, and the oxygen returned 
to the air. The carbon thus obtained unites with other 
substances brought in solution in the sap, thus manufactur¬ 
ing plant food and contributing to the plant’s growth. 
Dissolved in water, carbon dioxide contributes to the growth 
of aquatic plants. It is the most effective of the gases of 
the air in decreasing the intensity of the sun’s rays and in 
checking radiation of heat from the earth. 

Since plants use carbon dioxide in the daytime, it is well to 
have growing plants in the living room, the air on their 
account containing a slightly increased percentage of oxy¬ 
gen. On the other hand, they should be excluded from 
sleeping apartments at night, since they then use some of 
the oxygen and none of the carbon dioxide. 

When plants decay or are burned, the carbon stored up in 
their tissues is returned, usually, to the air in the form of 
carbon dioxide. Under certain conditions, however, as sub¬ 
mergence in water, or burial out of contact with the oxygen 


PROPERTIES AND FUNCTIONS OF THE AIR 85 

of the air, the carbon of the decaying plant may contribute 
to a future store of mineral fuel in the form of coal, oil, or gas. 

Function of Nitrogen. — Since nitrogen constitutes more 
than three-fourths of the weight of the air, without it the 
air would be less than one-fourth its present density. Flight 
for most forms would then be impossible, and moving air 
as an agent for driving machinery and wearing down the 
land would be correspondingly weakened. 

Another important function of nitrogen is its use as a 
plant food. It is a necessary element of the food of all 
plants, and like most other elements is taken through the 
roots in solution. If the soil is lacking in this element no 
plant will thrive. 

Unlike oxygen and carbon dioxide, nitrogen is not taken 
by the plant directly from the air as nitrogen , but comes by 
way of the soil from some soluble compound of nitrogen. 
Some nitrogen is obtained in the form of nitric acid , carried 
down from the air by falling rain drops. This supply was 
formerly supplemented chiefly by the application of fer¬ 
tilizers, often in the form of expensive nitrates imported 
from distant regions. 

Certain plants of the family to which the clovers belong 
are “ nitrogen gatherers.” These plants serve as hosts for 
minute organisms which, attaching themselves to the roots 
of the plant, gather and store upon the roots in little nodules 
the nitrogen from the ground air. Cowpeas, clovers, vetches, 
beans, and alfalfa are now extensively grown alike for their 
value as forage crops and for the nitrogen they add to the 
soil. Though the entire growth above ground may be re¬ 
moved, yet the soil will be left richer in nitrogen than 
before the crop was grown. 

We are now producing from the nitrogen of the air arti¬ 
ficial nitrogen compounds to supplement the natural ni¬ 
trates. These natural nitrates are found only in a few and 
regions and do not nearly supply the demand for nitrates. 
The great Muscle Shoals plant, started by the United States 


86 


NEW PHYSIOGRAPHY 


Government during the World War, was designed to manu¬ 
facture nitrates for war purposes while the war lasted, but 
in peace times for agricultural uses. A very high electric 
current is used to take the nitrogen out of the air. The 
nitrogen first combines with some of the oxygen, and nitrate 
fertilizer is made from this combination. Nitrogen, being 
required by all growing plants, is an essential element in the 
soil and must be restored to it either by nitrogen-gathering 
crops, such as clover, or in the form of nitrates. The un¬ 
limited supply of nitrogen in the air makes the process of 
nitrate manufacture of the greatest importance. 

Function of Water Vapor. — The water vapor of the air 
is the'source of clouds, fogs, and all forms of precipitation. 
Without it the earth would become parched, and life im¬ 
possible. It is lighter than dry air, and its presence makes 
the air lighter. Like carbon dioxide, it absorbs insolation 
(radiant energy from the sun) and heat radiated from the 
earth. Condensed as cloud, it is more effective in protecting 
the lands from the direct rays of the sun and in checking 
radiation of heat from the earth. Precipitated as rain, it 
supplies growing plants with necessary water, ahd as snow 
retards radiation and protects crops from the intense cold 
of winter. This function of snow is very important in the 
wheat-growing regions of the Northwest. 

In high altitudes and high latitudes where the precipita¬ 
tion is chiefly in the form of snow, melted snow is the prin¬ 
cipal source of water supply. 

Function of Dust. — Perhaps the most important function of 
dust is its diffusion (irregular scattering) of light. Without 
such diffusion objects would be visible only by reflection, as 
they now are at night; and the change from day to night and 
from night to day would be sudden, without twilight or dawn. 

At certain seasons a considerable part of the dust of the 
air is plant pollen. Many plants require pollen from other 
plants, and without the wind-borne pollen these would not 
be propagated. 


PROPERTIES AND FUNCTIONS OF THE AIR 87 


Putrefaction and fermentation are largely due to the 
organic dust of the air. Flesh and vegetables in high altitudes 
do not decay readily but simply dry out and shrivel up. This 
is due to the freedom of the air from dust germs. Some 
Indian tribes in these regions mummify their dead by simply 
exposing them in the air and sunshine. 

The germs of many diseases are distributed as air dust, 
and flesh wounds heal more readily when the dust germs 
are washed away and excluded from the wound. 

Every dust particle in the air is a nucleus about which 
water vapor may condense; consequently dust in the air 
promotes cloud formation and rainfall. Some have even 
taken the extreme view that without dust in the air no 
rainfall would be possible, but this has been disproved by 
experiment. 

Of aesthetic interest is the fact that the sky owes its 
beautiful and varying colors, for the most part, to the dust 
in the air. Gorgeous sunrises and sunsets occur when the 
air is laden with inorganic dust or with the smoke from 
forest fires. 

Origin of the Air. — If the nebular hypothesis concerning 
the origin of the solar system be accepted, the air may be 
considered a remnant of a formerly denser atmosphere. In 
this earlier atmosphere many of the elements which now 
make up the lands and seas existed as gases in an intensely 
hot condition. With loss of heat by radiation these elements 
changed to the liquid or solid state. Many of the elements 
of the primitive atmosphere were thus withdrawn, leaving 
the present remnant, the air as we know it. 

If we accept the planetesimal hypothesis of the origin of 
the solar system, we believe that the air has been driven 
out from the interior of the earth by the increasing tempera¬ 
ture and pressure. The gases thus driven out escaped to 
outer space while the earth was small and its gravitative 
attraction weak, and remained as part of the earth only 
after the earth’s attraction became strong enough to hold 
its gaseous envelope. 


88 


NEW PHYSIOGRAPHY 


Future of the Air. — Whatever the origin of our earth or 
of its gaseous envelope, the earth is continuously losing heat. 
We may therefore look forward to the time when it will have 
the temperature of outer space excepting only the surface 
that is turned toward the sun. 

Experiment proves that most gases can with sufficiently 
low temperatures be liquefied. We also learn from a study 
of the other members of our system besides the earth that 
the smaller ones, such as the earth’s moon, seem to have 
no atmosphere. These smaller members have cooled most; 
and if they ever had atmospheres, their present low tempera¬ 
tures have probably resulted in making their atmospheres 
part of their solid masses. 

We may therefore infer that, with further loss of heat by 
the earth, the terrestrial seas must in time become solid; and 
eventually the air itself will become in turn liquid and 
solid. Upon such an airless earth, life, as we know it, could 
not exist; and the earth would then appear, to an observer 
upon another planet, the lifeless globe that our moon now 
appears to us. 


QUESTIONS 

1. Why is it not correct to say “the air surrounds the earth”? 

2. How can you show that the air has weight? That it penetrates 
the soil? 

3. In what particulars is country air usually purer than city air? 

4. In what sense does rain purify the air? 

5. Why will plants thrive better than animals in hot, marshy 
lowlands? 

6. Why are trips to the mountains and sea voyages recommended 
for convalescents? 

7. How can you prove that there is dust in the air; and how 
can you decrease the dust in your bedroom without stopping 
ventilation? 

8. Why will milk that has been heated before bottling remain 
sweet longer than that which is bottled without heating? 

9. Why do dairymen cool their milk before shipping, and why 
is ice used to keep milk sweet? What is the principle of “cold 
storage”? 


PROPERTIES AND FUNCTIONS OF THE AIR 89 


10. Why will a candle lighted and lowered into a narrow deep 
bucket so quickly be extinguished? Why are lamp burners ven¬ 
tilated? 

11. Why do we ventilate our houses? What would be the result 
if we did not? Explain the horror of the “ Black Hole ” of Calcutta. 

12. How do you know there is water vapor in the air? 

13. Why should a wound be thoroughly cleansed before binding 
up? What is the principle of disinfection and sterilizationf 

14. Why is it necessary to dry our steel cutlery thoroughly, and 
not so necessary for silverware and china? 

15. Why do fires in open fireplaces and in stoves connected with 
flues burn better than fires built in the open air? 

16. How can the nitrogen of the soil be increased most economi¬ 
cally? 


CHAPTER VII 

TEMPERATURE OF THE AIR 

Sources of Heat. — So evident is it that the sun is the chief 
source of heat that the statement of the fact seems to need 
no demonstration. The days are warmer than the nights, 
and the temperature of our days increases with the increasing 
length of the period of sunshine and with the nearer approach 
of the noon sun to our zenith. Our coldest season is that in 
which the nights are longer than the days and when the 
sun’s noon position is low above the horizon. The hot belt 
of the earth is that which receives nearly vertical rays, while 
the frozen regions near the poles have only slanting rays. 

At first thought the sun appears to be the only source of 
heat; yet we know there are other sources. One of these 
minor sources, the interior heat of the earth, is of consider¬ 
able importance, notably in deep mines, and in the produc¬ 
tion of volcanoes and hot springs. 

The surface of the land varies in temperature from day to 
night and from summer to winter; but if we descend below 
the surface, the variation is less and less. A depth is finally 
reached, varying with the latitude, at which the temperature 
does not change, and below this depth the temperature 
grows warmer the deeper we go. On this account we con¬ 
clude that the interior of the earth is intensely hot. 

On the other hand, if we ascend in the air, we find that 
the temperature grows colder; and at the height of only a 
few miles freezing temperatures, even in summer, are reached. 
Reasoning from this basis we conclude that outer space 
is intensely cold. 

From our knowledge of cooling bodies we know then that 
the earth must he a cooling body, sending its heat in every 

90 


TEMPERATURE OF THE AIR 


91 


direction into outer space and bringing about equal amounts 
to every part of the surface of the land. 

Unimportant amounts of heat are received from stars, 
and reflected from the other planets and the moon. 

Insolation. — The radiant energy that comes to us from 
the sun is called insolation. It manifests itself in various 
ways, as heat, light, electricity, etc.; it is of varying wave 
lengths; and it travels at the enormously rapid rate of 
186,000 miles a second. Only that which is absorbed, by 
exciting molecular motion, manifests itself as heat. This 
alone warms the body. Heat may therefore be defined as 
the energy of molecular motion. 

As solar energy passes out from the sun center in all direc¬ 
tions, it is evident that only a very minute fraction of it will 
be intercepted by so small a body as the earth, at an average 
distance of about 93,000,000 miles. Of the amount thus 
intercepted, but a small portion is absorbed and transformed 
into heat; yet upon this minute part of the total solar en¬ 
ergy all of our life interests and activities depend. 

Disposal of Insolation. — When insolation is received, it 
is disposed of in three ways: by reflection, by transmission, 
and by absorption. As before stated, it is only the absorbed 
insolation that affects the temperature of the body. 

Each kind and condition of matter disposes of insolation 
in a distinct way. Some substances are good reflectors, 
some good transmitters, and some are good absorbers. Ex¬ 
periment has shown that in general, gases are the best trans¬ 
mitters, liquids the best reflectors, and solids the best absorbers. 

The following table sets forth, comparatively, the treat¬ 
ment of insolation received by land, water, and air: 



Land 

Water 

Air 

Reflector. 

Fair 

Good 

Very poor 

Transmitter. 

Poor 

Fair 

Very good 

Absorber. 

Good 

Poor 

Poor 














92 


NEW PHYSIOGRAPHY 


The absorptive power of a body may be materially modified 
by a change of color or of surface. Dark colors and irregular 
surfaces generally promote absorption, while light colors and 
smooth surfaces promote reflection. By increasing the re¬ 
flecting power of a body, we decrease its absorbing power. 

Loss of heat by radiation is in direct ratio to the absorbing 
power of a body, a good absorber being a good radiator and 
a poor absorber a poor radiator. If the reflecting power of 
a body be increased, its radiating power will be lessened. 
Radiation is continuous. 

Distribution of Heat. — Heat in a body may be dis¬ 
tributed by passing from particle to particle in contact; this 
process is called conduction. Solids are mainly heated in this 
way but differ widely in their power of conduction. 

In liquids and gases, e.g., water and air, the most impor¬ 
tant method of distributing heat is by convection. By this 
process particles in contact with a heated surface are warmed 
and expand. After expansion they are lighter, volume for 
volume, than the surrounding particles. The heavier parti¬ 
cles above then sink, under the greater pull of gravity; and 
the lighter are crowded upward away from the heating sur¬ 
face, the heavier being heated in turn. This process, depend¬ 
ing as it does upon gravity, requires that the heating surface 
be below the substance to be heated. 

The principle of convection is applied in the heating of 
our houses, whether by hot air, hot water, or steam. The 
heater is placed below the space to be heated, and radiators 
and hot-air flues are placed on or near the floor. 1 Flues and 
chimneys increase the movement of air over the fire by in¬ 
creasing the convectional motion in the air above. 

The land and water, being heated from above, are never 
warmed to very great depths; while the air, being chiefly 
heated at the bottom by contact with the land and water, is 
warmed more rapidly and through a much greater thickness. 

How the Air Is Heated. — The power of absorption of the 
air, though small, grows with increase in density, increase 


TEMPERATURE OF THE AIR 


93 


in carbon dioxide and water vapor, and increase in the num¬ 
ber of dust and liquid water particles present. Since each 
dust and water particle is a better absorber than air, it be¬ 
comes itself a center of warming. Therefore, when insolation 
comes to earth, it passes through the rare upper air with 
little loss by absorption. As it penetrates farther into the 
denser and dustier air, more and more of it is absorbed, and 
the air is more and more heated. The air absorbs from one- 
half to three-fifths, depending upon its cloudiness, of verti¬ 
cal insolation passing through it. 

The air is heated most at the bottom, not only because of 
the increased absorbing power of the lower layers, but also 
because of their contact with the warmer land and water 
surfaces. 

Another very important aid in the heating of the lower 
air is its convectional mixing. The heated lower air expands; 
and the cooler, heavier air above sinks and takes its place, 
to be in turn warmed and replaced by the cooler air above. 
As long as the lower air is lighter than the air above, con¬ 
vectional mixing will continue a factor in the warming of 
the lower air. This mixing is for the most part confined to 
the lower air, rarely reaching above seven miles in height. 
Above seven miles the temperature is about uniform; hence, 
in this region, known as the stratosphere , there is no convec¬ 
tion; only horizontal movement. 

The convectional ascent of heated air may be observed 
above a lighted gas jet, a hot stove, or a bonfire. Our rooms 
may be ventilated by admitting cool air at the bottom and 
permitting the escape of the heated air above. 

How the Air Is Cooled. — When insolation ceases, as at 
z night, conditions are reversed. Absorption, in excess of ra¬ 
diation during the day, is at night exceeded by radiation, 
and the air is cooled. Not that radiation does not continue 
during the day; for it is greatest when the temperature is 
highest, but the air does not begin to cool until radiation is 
more rapid than absorption. 


94 


NEW PHYSIOGRAPHY 


Since a good absorber is a good radiator, that part of the 
air which was most heated during the day is most cooled 
when insolation ceases. As a consequence, the rare, upper 
air is but little cooled, while the lower air is cooled most. 
Each dust and water particle, a center of warming during 
insolation, becomes a center of cooling when insolation 
ceases. 

One important factor in the warming of the air, convec¬ 
tion , is wanting when the air begins to cool. Because it is 
coolest at the bottom, the lower layers of air are heaviest; 
hence there is no tendency toward convectional mixing. In 
order to have cooling by convection, it would be necessary 
to have the air cooled most at the top. On this account the 
lower air warms up faster than it cools down. 

The coldest hour of the day is from 4 to 6 a.m., and the 
warmest from 1 to 3 p.m., depending upon the season. 
Thus it takes from seven to nine hours for the air to warm 
up, while from fifteen to seventeen hours are required for it 
to cool down. 

Temperatures Determined. — The temperature of the air, 
with reference to certain chosen temperatures, is determined 
by the thermometer. The temperatures of reference are 
those at which pure water freezes and boils under a pressure 
of approximately 14.7 pounds to the square inch. This is 
the average pressure of the air at sea level. 

The action of the thermometer is based on the fact that 
most substances expand uniformly with heating and contract 
uniformly with cooling. The measure of expansion or con¬ 
traction may be taken as a measure of the amount of heat¬ 
ing or cooling. 

Two general classes of thermometers are made, liquid and 
nonliquid. Almost any liquid or metal may be used, though 
in liquid thermometers mercury or alcohol is commonly 
employed. 

In the United States and other English-speaking countries, 
two scales for thermometers are in common use: the Fahren- 


TEMPERATURE OF THE AIR 


95 


heit (F.), and the Centigrade (C.). Their relation to each other 
and the method of converting readings of one to readings 
of the other are shown in the accompanying figure and table. 

Figure 38 shows both Fahrenheit and Centigrade scales. It will 
be observed that the two scales agree at — 40°. Freezing point is 
32° on the F., and 0° on the C., and boiling point 
212° and 100° respectively. It will thus be evident 
that a change from 32° to 212° on the F. ther¬ 
mometer is equivalent to a change from 0° to 100° 
on the C. This relation may be thus expressed: 

180° F. = 100° C. 

9° F. = 5° C. 

1° F. = f° C. 

I F. - 1° C. 

History of the Thermometer. — The thermometer 
was invented by Galileo early in the seventeenth 
century. Soon after its invention, it was graduated 
into 360 parts, corresponding to the number of 
degrees in a circle; hence the name degrees applied 
to these divisions. The name has been retained for 
the divisions of modern thermometers, though very 
differently and variously graduated. It was never 
significant. 

Fahrenheit was the first to adopt definite tempera¬ 
tures as a basis for graduation. According to his 
scale, the boiling point of water was found to be 212°, 
and the freezing point 32°. In the Centigrade ther¬ 
mometer 100° is taken as the boiling point and 0° 
the freezing point. 

The accuracy with which the instrument may be read depends 
upon the length of the degree, and this in turn depends upon the 
relative capacities of bulb and tube. It is essential to accuracy that 
the tube be of even bore. Why? 

Mercury and alcohol are commonly used in liquid thermometers, 
partly because of their even expansion at all ordinary temperatures, 
and partly because of their low freezing points. Mercury freezes 
at _ 40° F., and alcohol at about - 200° F. In the winter in high 
latitudes the temperatures are too low to be recorded by mercury 


c 

too 

F 

HZ 

90 _ 

194 

80_ 

176 

70_ 

158 

60_ 

140 

50_ 

122 

40 

104 

30 

_86 

10 _ 

_68 

10_ 

_SO 

0 

I3& 

-10 

_14 

-zo 

-4 

-30 

-22 

-40 

-40 




I 


Fig. 38. — F. 
AND C . 
Thermome¬ 
ter 





96 


NEW PHYSIOGRAPHY 


thermometers. On the other hand, mercury is the better suited for 
high temperatures, since its boiling point is 660° F., while that of 
alcohol is only about 173° F., or lower than the boiling point of 
water. 

Maximum Thermometer. — It is often desirable to know 
the highest temperature attained during a given period. For 
_ _ this purpose the maximum 

thermometer is used. This is 
a modification of the ordinary 
liquid thermometer by a slight 
constrict on in the bore just 
above the bulb. This narrowed 
bore, though wide enough to 
allow the expanding liquid to 
press through, is too narrow 
for the liquid column, of its 
own weight, to pass back as 
the temperature falls. The 
thermometer thus continues to 
indicate the highest tempera¬ 
ture attained. 

The clinical thermometer 
used by physicians is a maxi¬ 
mum thermometer. To set 
the instrument for a new read¬ 
ing, the column of liquid must 
be made to unite by swinging 
or jarring the instrument. 

Minimum Thermometer. — 
The minimum thermometer for 
registering.lowest temperatures is simply an ordinary alcohol 
thermometer, with colorless liquid, containing a short double¬ 
headed pin. The heads of the pin are slightly smaller than 
the bore, in order that the alcohol may pass by the pin. 

For registering a minimum temperature the tube is placed in an 
inclined position, so that gravity cannot pull the pin down the tube; 




Fig. 39.—Max¬ 
imum Ther¬ 
mometer 


Fig. 40.—Mini¬ 
mum Ther¬ 
mometer 












TEMPERATURE OF THE AIR 


97 


but when gravity is assisted by the surface tension of the liquid, 
when the upper end of the contracting column comes in contact with 
the upper head of the pin, the pin is pulled down the tube. When 
with rising temperature the liquid column begins to lengthen, it 
passes over and by the pin, but cannot push the pin against gravity 
up the tube. The upper end of the pin thus registers the lowest or 
minimum temperature attained. 

To set the instrument for registering a new minimum the ther¬ 
mometer is held, bulb upward, until the pin sinks through the liquid 
to the end of the column. The instrument is then placed in the in¬ 
clined position in which it ordinarily rests. 

Thermograph. — To obtain a continuous record of the 
temperature, a self-registering thermometer, or thermograph , 
is used. The varying temperature is recorded by a pen, 
moved by a system of levers. The pen rests against a disk 
or cylinder of paper, which is moved by clockwork. A con¬ 
tinuous mark is made by the pen. By reference to two sets 
of lines ruled upon the disk or sheet (temperature lines and 
time lines), this mark gives a continuous record of tempera¬ 
ture. The thermograph takes the place of the maximum 
and minimum thermometers. 

The record made by the thermograph is a thermogram or 
temperature curve for the period of time covered by the 
record. (See Figure 42.) 

Approximately accurate temperature curves may be made 
from observations of the thermometer taken every two 
hours. From the daily averages monthly curves and from 
the monthly averages, annual temperature curves may be 
constructed. 

Distribution of Insolation. — The amount of insolation 
received by a given area of land or water in a given time, as 
during one complete rotation of the earth, depends mainly 
upon the following variables: 

1. Length of insolation period or the number of hours of sunshine. 

2. The angle at which the noon insolation ray is received. 

3. Condition of the air with regard to dust and cloudiness. 


98 


NEW PHYSIOGRAPHY 


4. The length of the path of the ray through the atmosphere. 

5. The distance from the sun. 

Length of Insolation Period. — Because the earth’s axis is 
inclined to the plane of its orbit, the insolation period is not 

the same for all places, 
nor for the same place at 
all times. Most places 
upon the earth have the 
period of rotation un¬ 
equally divided between 
sunshine and shadow. 

At the equator the 
period of insolation is 
always 12 hours. In all 
other latitudes it is only 
at the equinoxes that the 
insolation period is 12 hours; being longer than 12 hours when 
the sun is on the same side of the equator as the observer, 
and shorter than 12 hours when the sun and observer are on 
opposite sides of the equator. 

The higher the latitude, the greater the length of the con¬ 
tinuous insolation period. Within the polar circles it varies 



Fig. 42. — Thermograph Record for One Week 
Note daily variation of temperature and the hours of highest and lowest temperature. 



Fig. 41. — Thermograph 


from no insolation in midwinter to 24 hours of insolation 
in midsummer , for each rotation. 

Other things being equal, the amount of insolation re¬ 
ceived varies as the length of the insolation period. There 
is, therefore, at summer solstice a constantly increasing 




















TEMPERATURE OF THE AIR 


99 


amount of insolation, during one rotation, from the equator 
to the polar circle of the summer hemisphere and a con¬ 
stantly decreasing amount from the equator to the polar 
circle of the winter hemisphere. 


Relation of Latitude to Greatest Length of Day or Night 


Latitude 

Greatest Length of 
Day or Night 

Latitude 

Greatest Length of 
Day or Niget 

Q° 

12 hr. 

00 min. 

50° 

16 hr*. 

04 min. 

5° 

12 

“ 

16 “ 

55° 

17 “ 

00 “ 

10° 

12 

“ 

40 “ 

60° 

18 “ 

15 “ 

15° 

12 

“ 

52 “ 

65° 

20 “ 

48 “ 

20° 

13 

“ 

12 “ 

66.5° 

24 “ 

00 “ 

25° * 

13 

“ 

34 “ 

70° 

64 days 

30° 

13 

“ 

54 “ 

75° 

103 “ 


35° 

14 

“ 

20 “ 

80° 

133 “ 


40° 

14 

“ 

48 “ 

85° 

160 “ 


45° 

15 


20 “ 

90° 

187 “ 



Angle of Insolation. — Since the earth’s shape is globular, 
the angle at which the sun’s ray strikes is different at differ¬ 
ent places. There is one vertical ray, which shifts from day 
to day, being at the equator at the equinoxes and at the 
tropics alternately at the solstices. Outside the tropics 
there is never vertical insolation, although the slope of the 
land may be such as to give perpendicular insolation any¬ 
where or at any time of day. On a horizontal surface the 
angle of insolation is zero at sunrise and sunset and greatest 
at noon, changing continually during the day. 

The angle at which the sun’s ray strikes any place will 
therefore vary with the time of day, the time of year, the 
latitude of the place, and the slope of the land. 

Because of the inclination of the earth’s axis and revolu¬ 
tion, the angle of the sun’s noon ray varies at any station from 
day to day. Vertical noon insolation occurs at the equator at 
the times of the equinoxes 5 and at the tropics, alternately, 
at the times of the solstices. During the year vertical noon 
insolation occurs twice at every station within the belt, 














100 NEW PHYSIOGRAPHY 

47° wide, lying between the tropics. This belt is some¬ 
times called the torrid zone. No place outside this zone 
ever receives vertical insolation, the maximum angle being less 
i 





Fig. 43. — Amount of Insolation Received on Surface AB; 
I, Perpendicular; II, Inclined 


and less with increase of latitude, reaching 23J° at the poles. 

Hence, in so far as the angle of insolation determines the 

amount of insolation 
received during one 
rotation, the maxi¬ 
mum amount is al¬ 
ways received upon 
or between the trop¬ 
ics. The average for 
the year is greatest at 
the equator and least 
at the poles. 

In the above Figure 
43, AB is a surface 
receiving insolation at different angles. Since beam I has the cross- 
section AB and beam II the cross-section BC, the surface AB 
receives the greatest amount of insolation when the beam strikes 
perpendicularly. The same is true in Figure 44 where the beam 
is shown striking two surfaces, MN and MR, at different angles. 
Since the beam is concentrated upon a smaller surface when it 
strikes perpendicularly than when it strikes obliquely, the perpen- 



Surface Inclined to Beam of Insolation 





























TEMPERATURE OF THE AIR 


101 


dicular insolation is the more intense. In Figure 43 the surface 
is constant and the cross-section of the beam variable; in Figure 44 
the beam is constant and the surface is variable. 

Condition of the Air. — The most variable constituents of 
the air are likewise those which most intercept insolation. 
These are, in the order of their importance, cloud particles 
and dust. Clouds and dust in the air intercept insolation 
and thus prevent land and water surfaces from being as 
much heated as they would otherwise be. They also intercept 
radiant heat. For this reason those places where cloudiness 
prevails have a more constant temperature than places with 
prevailingly clear skies. Cloudy days are less warm in sum¬ 
mer and less cold in winter than are clear days, and the insola¬ 
tion on the mountain top is more intense than in the valley. 

Length of Path through Air. — Oblique rays pass through 
a greater thickness of air than do vertical rays; and whereas 
vertical rays lose half of their intensity, rays approaching 
tangency lose more than 90 per cent. 


Intensity of Insolation at Different Angles 


Altitude of 
the Sun 

Relative Length of 
Path of Ray through 
Atmosphere 

Intensity of Inso¬ 
lation on Surface 
Perpendicular 
to Rays 

Intensity of In¬ 
solation ON A 
Horizontal 
Surface 

0° 

44.70 

0.00 

0.00 

10° 

5.70 

0.31 

0.05 

20° 

2.92 

0.51 

0.17 

30° 

2.00 

0.62 

0.31 

40° 

1.56 

0.68 

0.44 

50° 

1.31 

0.72 

0.55 

60° 

1.15 

0.75 

0.65 

70° 

1.06 

0.76 

0.72 

80° 

1.02 

0.77 

0.76 

90° 

1.00 

0.78 

0.78 


While the poles alternately receive more insolation than 
any other portion of the earth, for a brief period about the 
summer and winter solstices respectively, owing to con¬ 
tinuous insolation there, all the conditions combine to give 









102 


NEW PHYSIOGRAPHY 


to places at the equator about two and one-half times the 
amount of insolation annually received at the poles. 

Figure 45 illustrates the varying length of the ray through the 
air with varying angle. AB represents a section of the solid earth 
and CD the outer surface of the air above it. An observer at 0 is 
receiving the vertical ray 1, an oblique ray #, and a horizontal 
ray 8. It is evident that the greater the obliquity, the longer the 
path of the ray through the air, and consequently the less intense 
the insolation. 


Distance from Sun. — The amount of insolation received 
varies inversely as the square of the earth’s distance from 



the sun. While this factor has a scarcely perceptible value 
as between any two places upon the earth, at any given 
time, the difference in distance being never as much as 
4000 miles, yet as between winter and summer the value 
is considerable. The earth is about 3,000,000 miles nearer 
the sun at perihelion, about January 1, than at aphelion, 
about July 1. In consequence a place receiving vertical 
insolation January 1 receives about 5 per cent more inso¬ 
lation than one receiving vertical insolation July 1. 

Distribution of Heat over the Earth. — The distribution 
of heat over the earth does not agree with the distribution 
of insolation, though in general following it. It should be 





TEMPERATURE OF THE AIR 103 

remembered that heat is caused by absorbed insolation, and 
whatever factors enter into the control of absorption to that 
extent affect the temperature of the absorbing substance. 

Increasing obliquity, or slant, of the sun’s rays is accom¬ 
panied by a more rapid decrease of heat developed than of in¬ 
solation received. It results in an increased percentage of 
insolation reflected and consequently a decreased percentage 
absorbed. It is found that while water reflects only two 
per cent of vertical insolation, it reflects about 65 per cent 
when the sun is only 10° above the horizon. On this ac¬ 
count the early morning and late afternoon rays and the 
rays received in high latitudes have little effect in increas¬ 
ing temperatures. For this reason alone the polar regions 
could never be warm; and the low temperatures reported 
by our Arctic and Antarctic explorers as occurring there in 
midsummer are in part accounted for. 

When we consider also the fact that in the polar regions 
the lands and frozen seas are for much of the year covered 
with snow and ice, both very poor absorbers, and that the 
heat produced by absorption must first be used to melt the 
snow and ice, we may better appreciate the low temperatures 
which prevail there. 

The northern hemisphere, where the continents are 
massed, is warmer in summer and colder in winter than the 
southern hemisphere, which is mostly water. This is due to 
the fact that land is a better absorber and better radiator 
than water and the further fact that it requires more heat to 
warm the water than the land. 

Dark-colored rocks and soils, because they are better ab¬ 
sorbers than light-colored ones, are warmer under sunshine 
and colder when insolation is withdrawn. This in a measure 
controls the character and amount of plant growth and 
affects the distribution of heat. 

The direction and character of winds and ocean currents 
— to be explained — and the differences in the altitude of 
the land are likewise important factors in the distribution 


104 


NEW PHYSIOGRAPHY 


of heat over the earth, though not affecting the distribution 
of insolation. 

All things combine to give to regions along the equator 
the greatest total amount of heat and to make its distribu¬ 
tion through the year most equable there. 

Shifting of Heat Equator. — This zone of greatest heat, 
the hot belt, near the geographical equator — of varying 
width — is known as the doldrum belt , or simply the doldrums. 
The line in the midst of this belt, passing through places 
having the highest temperatures, is called the heat equator. 

Since the sun’s vertical ray shifts during the year over a 
zone 47° wide, so the doldrums and heat equator shift, 
though over a narrower zone. 

The temperature of a place continues to increase so long 
as more heat is received than is lost by radiation. The 
cooling down begins, ordinarily, an hour or two past noon, 
though most heat is received at noon; and the highest 
temperature of the year occurs usually some weeks after 
the longest day, although most heat is received on that 
day. 

The doldrum belt and heat equator, therefore, do not 
attain their extreme positions north and south at the times 
of the solstices, but weeks after. Places between the tropics, 
having vertical insolation twice annually, have two maxima 
and two minima temperatures during the year, and expe¬ 
rience their highest maximum temperature shortly after 
vertical insolation upon the sun’s return toward the 
equator. 

Average Position of Heat Equator. — The heat equator 
shifts farther and remains for a longer time north of the 
terrestrial equator than it does south of it. This is in part 
because the sun is seven days longer north of the equator 
than south of it and in further part because of the forms of 
the continents and ocean basins. Owing to the positions and 
outlines of the continents, more of the warm ocean currents 
are turned into the northern oceans than into the southern. 


TEMPERATURE OF THE AIR 


and these make the northern hemisphere on an averageime 
warmer. 

Moreover, the Pacific basin, being almost closed at the 
north, thus practically shutting out the cold polar currents 
that freely enter the North Atlantic, makes the North 
Pacific a warmer ocean than the North Atlantic. The aver¬ 
age position of the heat equator is, therefore, more northerly 
in the Pacific than in the Atlantic. 

Shifting Most over Atlantic. — Being a better absorber 
and better radiator than water, land has a higher tempera¬ 
ture in summer and a lower temperature in winter than the 
sea in the same latitude. This excessive warming and 
cooling is most pronounced in its effects in the northern 
hemisphere, where the great land areas are, and is also more 
pronounced over the relatively narrow Atlantic than over 
the broader Pacific. 

Isotherms. — Lines drawn through places having the 
same temperature at the same time are called isotherms. 
They may represent the distribution of temperatures at any 
given time, or they may represent the averages for any de¬ 
sired period — a week, a month, or the entire year. Such 
lines, while very irregular, have in the main a general east- 
west direction. This is what we should expect, inasmuch as 
length of insolation period and angle of insolation, the most 
important factors in determining the distribution of heat, 
are constant along any given parallel. The minor factors 
in the distribution of heat are responsible for the departure 
of isotherms from the parallels. 

Isotherms are continuous lines which for a limited area 
may appear upon the map as closed curves. From their 
definition two isotherms cannot intersect. The heat equator 
is not an isotherm, though it extends around the earth in 
the same general direction as isotherms. It may cross 
isotherms. 

Temperature Gradient. — If we pass from one isotherm 
to the next of higher or lower temperature, we must pass 



106 


NEW PHYSIOGRAPHY 



Fig. 46 . — Isotherms and Heat Equator of the World for January 

Observe that the heat equator lies for most part south of the geographic equator, though nearer to it toward the 

eastern side of oceans, and actual north of it in the eastern Atlantic. 





















































































































































120 140 160 180 160 140 120 100 80 60 40 20 0 20 40 60 80 100 120 


TEMPERATURE OF THE AIR 



107 


Fig. 47. — Isotherms and Heat Equator for July 
Observe that the heat equator lies mostly north of the geographic equator and farthest north over continents. 











































































































































108 


NEW PHYSIOGRAPHY 


through all intermediate temperatures. While we may pass 
along an indefinite number of routes, it is evident that the 
shorter the route the more rapid the change of temperature. 
The shortest route, which gives the maximum rate of change, 
is the direction of the temperature gradient. 

Temperature gradient may be defined as the rate of change 
of temperature between two places. The gradient at any place 
is the steepest gradient or the gradient in the direction of 
most rapid change. 

The more closely the isotherms are crowded, the more 
rapid the change of temperature, or as we say, the steeper 
the gradient , while widely separated isotherms indicate gentle 
gradients. 

Isothermal Charts. — If the isotherms of any region be 
drawn, the result is an isothermal chart. Daily, monthly, 
seasonal, and annual charts are commonly made. Isothermal 
charts are graphic representations of temperature readings 
where time is constant and place variable; whereas tempera¬ 
ture curves are records with place constant and time variable. 

Vertical Distribution of Heat. — If we ascend through 
quiet air, as in a balloon, we shall find that, as a rule, the 
temperature of the air decreases; descending, the tempera¬ 
ture increases. This change, due to difference of altitude, 
is about 1 ° F. for every 300 feet. It is known as the vertical 
temperature gradient. 


QUESTIONS 

1 . If the interior heat of the earth were the chief source of heat, 
what part of the earth’s surface would be hottest? 

2. What reason have you for thinking that the sun, rather than 
some other outside source, is the chief source of our heat? 

3. What percentage of the sun’s radiant energy is received by 
the earth? Does Mercury, Venus, or the earth receive the largest 
percentage? 

4. Why are dark shades of clothing better suited to winter than 
to summer? Why are dark-colored soils earlier ready for seeding 
than light-colored? 


TEMPERATURE OF THE AIR 


109 


5. Why do we heat our kettles from below, and why place the 
radiators that heat our rooms near the floor rather than near the 
ceiling? 

6 . Aviators always find the air cold at the height of a few 
thousand feet. Why is this? 

7. The higher we ascend in the air the more intense the insola¬ 
tion yet the lower the temperature. Why is this? 

8 . Why do lakes and rivers cool down so much more quickly 
than they warm up? Why do shallow lakes freeze over more quickly 
than deep ones? 

9. Why is the temperature of Denver more equable than that 
of St. Louis? Chicago more equable than Minneapolis? 

10. Why is a uniform bore necessary in the tube of an accurate 
thermometer? Why is the tube expanded at the bottom? 

11. How can you use the thermometer to determine altitude? 

, 12. Why, in the northern hemisphere, do flowers bloom and 
trees put forth their leaves so much earlier on the southern slopes 
than upon the northern slopes of hills and mountains? 

✓13. Why is a cloudy day in winter warmer, and in summer cooler, 
than a clear day? 

14. Why is it warmer in summer in the latitude of St. Louis than 
at the equator? 

15. Why is the warmest hour of the day later in summer than in 
winter, and why is the coldest hour earlier? 

16. Why is there less difference between the two maxima tem¬ 
peratures during the year than between the two minima, at places 
over which the vertical ray of the sun shifts? 


CHAPTER VIII 

WEIGHT AND DENSITY OF THE AIR 

Pressure of the Air. — It is a well-known fact that, at 
any point within a liquid or a gas, pressure is equal in all 
directions. On this account one moves freely about in the 
air, although it is pressing upon every square inch of the 
body with a pressure of almost 15 pounds, or more than 
a ton to the square foot. Nevertheless, this great pressure 
causes us no inconvenience, because it is balanced by an 
equal pressure from within. Pressure of the air is pressure 
per unit area. 

The pressure of the air at sea level sustains a vertical 
column of fresh water about 34 feet high, and a vertical col¬ 
umn of mercury about 30 inches high. This fact is applied in 
the lifting pump, the siphon, and the barometer. 

Weight of the Air. — At sea level a cubic foot of air 
weighs 1.25 ounces; and in a room 14 feet long, 12 feet 
wide, and 10 feet high, there are 131 pounds of air. The 
weight of the air above an acre of ground is almost 50,000 
tons. The weight of the air above any horizontal surface is 
equal to the pressure upon that surface, weight being simply 
pressure downward. 

Density of the Air. — In gases, pressure, density, and 
volume bear a definite relation to each other. As the pressure 
increases the density also increases, and the volume decreases 
in the same ratio. This is not true of liquids or solids. As 
a result of this relation the air is densest at the bottom. 

So rapidly does the density of the air decrease as we ascend 
in it that at an altitude of about 3.1 miles the air is only half 
as dense as at sea level. This means that half of the air is 

110 


WEIGHT AND DENSITY OF THE AIR 111 

within 3.1 miles of the surface of the sea; and since many 
mountains are more than three miles high, their summits 
reach above one-half of the entire mass of the air. Within 
the next three miles we pass through almost one-half of the 
remaining half of the air; so that three-quarters of the air 
is within 6.2 miles of the surface of the sea. If 
the air were of the uniform density of the lower 
air, it would extend only about five miles above 
sea level. 

Measurement of Pressure. — For the purpose 
of measuring the pressure of the air the 
barometer has been devised. Its construction 
depends upon the principle that a given weight 
of air will balance an equal weight of any other 
fluid, or counterbalance an equal pressure 
exerted in any other way. 

Two types of barometer are in common use, 
the liquid and nonliquid. In the liquid 
barometer the air sustains a column of liquid, 
commonly mercury, in a tube from which the 
air has been withdrawn. In the nonliquid or 
aneroid barometer the pressure of the air is 
counterbalanced by a spring beneath a thin 
metal diaphragm. 

The Mercury Barometer. — The simple fig. 48 . —s™- 

J . j.* n r ple Barome- 

mercury barometer consists essentially ol a TE r 
glass tube about 34 inches long, closed at one 
end, filled with mercury, and placed vertically, open end 
down, in a cistern of mercury, and a scale. The mercury 
sinks in the tube, leaving a few inches at the upper end of 
the tube a vacuum, consequently with no pressure on the 
mercury column. The column of mercury in the tube is 
supported by the pressure of the air upon the open surface 
of the mercury in the cistern. When this pressure increases, 
the mercury rises in the tube, or the barometer is said to 
rise; and when the pressure decreases, the barometer falls. 



112 


NEW PHYSIOGRAPHY 


The tube is graduated in some linear unit, as the milli¬ 
meter or tenth of an inch, the surface of the mercury in 
the cistern being the zero of the scale. 

At sea level the length of the mercury column is about 
30 inches; hence we commonly say the normal pressure at 
sea level is 30 inches, understanding that the 
pressure is measured by the weight of the 
column of this length, as pressure cannot be 
measured in inches. 

As it is the vertical length of the column of 
mercury that measures the pressure of the air, 
it is necessary, when taking a reading, to hold 
the instrument in a vertical position. For this 
purpose and to protect the instrument, the tube 
and cistern are firmly bound together to a rigid 
frame, arranged for suspension. 

The Aneroid Barometer. — The aneroid 
barometer consists of a pile of hollow metallic 
disks, from which the air is exhausted and to 
which an index is attached. This index moves 
over a surface upon which there are graduations 
to represent the various pressures. The disks 
are made of very thin metal, supported by 
springs within, and these disks respond to slight 
changes in air pressure. Because of its con¬ 
venience, the aneroid is much used in taking 
altitudes. Both pressure in inches and alti¬ 
tudes in feet are usually shown. 

Variation in Barometer Reading. — At sea 
level, as we have seen, the average reading of 
the barometer is about 30 inches. As the instrument is 
carried up through the air, in a balloon or in ascending a 
mountain, it is found that the barometer reading is lower by 
about one inch for each thousand feet of ascent. This is 
because of the air that is left below, only the air above 
affecting the barometer. 



Fio. 49.— 
Standard 
Barometer 











WEIGHT AND DENSITY OF THE AIR 


113 


This is only approximately true, for with increase in alti¬ 
tude there is a decrease in the density of the air. Whereas 
a fall of one inch results from carrying the instrument from 
sea level up 910 feet, a fall of two inches requires an ascent 
of 1850 feet. The higher the altitude, the greater the dis¬ 
tance through which the instrument must be carried to 
register a fall of one inch. 

Height of the Air. — Estimates of the thickness of the air 
envelope based upon barometer readings are unreliable, in¬ 
asmuch as we do not know at what rate 
the density of the upper air changes. 

While one-half of the air lies within 3.1 
miles above sea level, we have reason to 
know that the air in considerable density 
exists at a height of more than 200 miles. 

Meteors have been observed at that 
height. 

Lows and Highs. — If a stationary 
barometer is read from hour to hour, it 
will be noted that its readings vary 
continuously. This seems to be due chiefly to a succession of 
surges in the air, called lows or highs as the barometer falls or 
rises. Lows are also called cyclones and highs anticyclones. 

As we go outward from the center of a low, we observe 
that the barometer readings are higher in all directions; 
and in passing out from the center of a high, that the read- 



Fig. 50. — Aneroid 
Barometer 


ings are lower. It follows that about lows and highs systems 
of lines may be drawn through places having the same barom¬ 
eter reading. Lines drawn through places having the same 
barometer readings at the same time are called isobars. 

Because of the mobility of the air and the many conditions 
that affect pressure, such as variation in temperature and 
in the amount of water vapor in the air, isobars are not 
likely to be either regular or parallel. About strongly de- 
yeloped lows and highs the isobars are closed curves and 
approximately parallel. 


114 


NEW PHYSIOGRAPHY 


The isobars about a high may be aptly likened to the con¬ 
tours of a hill in a topographic map, the high by analogy 
being an atmospheric hill; and those about a low may be 
likened to the contours of a depression , the low being an 
atmospheric basin. 

Inasmuch as the density varies directly as the pressure, 
the air is denser about a high than about a low. 

Pressure Gradient. — Just as the temperature gradient 
line is the shortest distance from one isotherm to the next, 
so we may get the pressure gradient line at any place by 
taking the shortest distance between the isobars at that 
place. 

The pressure gradient is the rate of change of pressure be¬ 
tween two places; and, like the temperature gradient, at any 



Fig. 51. — Isobaks about a Low and a High 


place it is the direction of the most rapid change. Crowded 
isobars, therefore, signify steep pressure gradients, and wide¬ 
spread isobars gentle gradients. We shall see that the direc¬ 
tion and strength of the wind are closely related to the 
pressure gradient. 

If a continuous record of the air pressure is desired, an in¬ 
strument called the barograph is used. It is usually an aneroid 
barometer with a pen-bearing arm in the place of the index. 
The pen point rests against a disk or sheet of paper that 


WEIGHT AND DENSITY OF THE AIR 


115 


moves at a constant rate, as in the thermograph. The 
two systems of lines are time and pressure lines. 

The record of a barograph is called a barogram or pressure 
curve; and the charted pressures of any region, as shown by 
the isobars for that region, is a 
pressure or isobaric chart. 

Pressure Belts. — The distri¬ 
bution of pressure over the earth 
is intimately associated with the 
distribution of temperature; and 
while the isobars do not have as 
decisive an east-west trend as 
the isotherms do, in general the two systems of lines for the 
world are parallel. 

Because of the high temperature, the equatorial belt is 



Fig. 52. — Barograph 



Fig. 53. — Barograph and Thermograph Records for One Week 


a region of low barometric pressure. This region is known as 
the doldrums. 

The air thus crowded out from above the doldrums is 
piled up on either side, giving in each hemisphere a belt of 















































116 


NEW PHYSIOGRAPHY 



summer hemisphere the opposite is true. 









































































WEIGHT AND DENSITY OF THE AIR 


117 



Fig. 54 B . — Distribution of Pressure During the Northern Summer 
Now the continents in the northern hemisphere are low-pressure centers and the oceans high. 

















































































118 


NEW PHYSIOGRAPHY 


high pressure called the horse latitudes. They lie in about 
latitude 30 north and south. 

Poleward from the horse latitudes the pressure decreases, 
and the polar areas are thought to be areas of relatively low 
pressure. 

As a result of this arrangement of pressure, the isobars 
of the world have a general east-west trend and shift with 
the shifting belt of equatorial heat. 

Uses of the Barometer. — Much the most important use 
of the barometer is in forecasting the weather. Pressure is 
one of the factors which determine the weather; and in fore¬ 
casting, a knowledge of the distribution of pressure over the 
country is necessary. 

Because of its convenience in carrying, the aneroid ba¬ 
rometer is used in determining altitudes. Balloonists, 
aviators, and mountain climbers carry aneroids, this being 
the best means by which they may find the altitude reached. 

QUESTIONS 

1. In a closed vessel filled with air, the pressure upon the inner 
surface decreases with decrease of temperature, whereas in the open 
air pressure increases with decrease of temperature. Why is this? 

2. What is meant when we say the pressure of the air is 30 
inches? 

3. Why does the air at any place vary in pressure? 

4. How may the barometer be used to measure altitude? 

5. Why must a liquid barometer be held in a vertical position 
when read? Why is it not necessary to hold an aneroid in a definite 
position? 

6. Why is it not necessary that the bore of the barometer tube 
be regular as in the thermometer? 

7. What is the general relation between barometer change and 
change of thermometer? 

8. Why is mercury so generally used in the construction of 
liquid barometers? What is the objection to using water? 

9. Why do standard barometers have a thermometer attached? 

10. Why do the high pressure belts of the horse latitudes shift? 


CHAPTER IX 


MOVEMENTS OF THE AIR 

Winds Defined and Explained. — Since the air is part 
of the earth, by necessity it partakes of the earth’s motions 
of rotation and revolution. Entirely distinct from these 
motions are those sometimes regular, but more often fitful 
and irregular, movements of the lower air called winds. 

A wind may be defined as an approximately horizontal 



Fig. 55. —-Showing How Winds Are Started 

Numbers below AB indicate barometer reading before heating; 
those above AB barometer reading after heating. 


natural movement of the lower air. Winds should be sharply 
distinguished from vertically moving air; also from the 
upper-air movements, both of which are called currents. 

The air is so mobile that any object moving through it 
sets a considerable volume in motion, and the least inequality 
of pressure disturbs its equilibrium. The most important 

119 


















120 


NEW PHYSIOGRAPHY 


cause of the unequal distribution of pressure, and therefore 
of winds, is the unequal distribution of heat over the earth. 

In Figure 55 AB represents any surface above which the air ex¬ 
tends to the height GH. Suppose a limited area, CD, is heated in 
excess of the surrounding surface. The air above CD expands; and 
if we consider only the upward expansion, the column of air above 
CD lengthens to KL. But being unsupported laterally, the air in 
the column above the level of GH spreads out until the upper surface 
of the air is again level at the height of EF. Some air above CD 
having flowed away, the pressure upon this surface is less than before 
heating; whereas the pressure upon AC and BD is greater than 
before heating. 

As a result of the rearrangement of pressures, AC and BD are 
highs. The lower air moves laterally away from AC and BD, 
chiefly toward the low CD. Above CD the inflowing currents meet 
and rise, thus completing the circulation. The circulation continues 
just so long as the area CD is heated in excess of the adjacent areas. 

If we consider the separate steps in the development of the cir¬ 
culation described, they may be stated as: 

1. Local excessive heating. 

2. Expansion of the column of air above the heated area. 

3. Overflow aloft from and inflow at the bottom toward the heated 
area. 

4. Ascent of the inflowing currents above the heated area, or low. 

5. Descent of the upper currents that move away from the low 
center. 

It is only the surface movement in this circulation that we usually 
call winds; all other parts of the circulation are properly called 
currents. 

Terrestrial Winds. — If we consider the foregoing figure 
a vertical section of the air along a meridian at the equator, 
we have an explanation of the systematic winds of the earth. 
The area CD represents the doldrum belt at or near the equa¬ 
tor, which, by reason of nearly vertical insolation, is most 
heated. The movement of the air above this belt, being 
chiefly upward, makes this a belt of light winds or calms. 
These are sometimes called the equatorial calms. 


MOVEMENTS OF THE AIR 


121 


The poleward overflow aloft leaves this region a belt of 
low pressure, and at the same time produces on either side, 
somewhere between the latitudes of 25° and 35°, a belt of 
high pressure. Above these belts of high pressure the move¬ 
ment of the air is chiefly downward; and these, like the 
doldrum belt, are also belts of light winds or calms, the 
so-called horse latitudes spoken of before. Out from these 
high-pressure belts the winds blow toward the equator, and 
with less pronounced strength toward the poles. 

Excessive heating along a belt near the equator results 
from: (1) the globular shape of the earth, (2) rotation about 
an dxis which remains parallel to itself, (3) the source of heat 
being a body distant from the earth. 

Since the planets agree in these three particulars, it fol¬ 
lows that the circulation described for the earth must be 
common to all planets with atmospheres. On this account 
the winds produced by this circulation are sometimes called 
planetary winds. 

The accompanying tables give the approximate positions 
of the doldrums and horse latitudes in both the Atlantic and 
Pacific Oceans for March and September. Owing to the incli¬ 
nation of the earth’s axis, these belts shift and occupy their 
extreme positions in the months named. 


Atlantic Ocean 


March 

September 

N.E. Trades. 

26° N. to 3° N. 

35° N. to 11° N. 

Doldrums. 

3° N. to 0° N. 

11° N. to 3° N. 

S.E. Trades. 

25° S. to 0° N. 

25° S. to 3° N. 


Pacific Ocean 



March 

September 

N.E. Trades. 

Doldrums. 

S.E. Trades. 

25° N. to 5° N. 

5° N. to 3° N. 

28° S. to 3° N. 

30° N. to 10° N. 

10° N. to 7° N. 

20° S. to 7° N. 
























122 


NEW PHYSIOGRAPHY 


Deflection of Winds. — It was long ago discovered that 
winds do not follow a straight course, but in the northern 
hemisphere turn to the right and in the southern hemisphere 
to the left of such a course. The statement of this system¬ 
atic deflections is known as FerreFs Law, and all winds 
follow this law. It governs alike the constant winds that 
move away from the high-pressure horse latitude belts, and 
the irregular winds that blow about high- and low-pressure 
areas. Because of this deflection winds do not follow the 



Fig. 56. — The Deflection of the Winds from a Straight Course, 
Because of the Rotation of the Earth 


barometric gradient. This deflection results from the rota¬ 
tion of the earth. 

In explanation of the relation between deflection of the 
winds and rotation, two facts are important: 

1. The rotational velocity of places upon the earth decreases 
poleward. 

2. The inertia of matter makes it impossible for a particle 
once set in motion of itself to change its direction of motion. 

Figure 56 illustrates an experiment with an inked marble and a 
rotating table. The results of this experiment are closely similar 
to winds moving upon a rotating earth. 

In I the marble is started from the center along the radius CA, 
with sufficient velocity to carry it to the edge of the table in one 





MOVEMENTS OF THE AIR 


123 


second. If the table is at rest, the marble will leave the table at A. 
If, however, at the instant the marble is set in motion the table is 
set rotating in a counterclockwise direction at such a rate as to bring 
CA to CA' in one second, the marble will follow a curved path, leav¬ 
ing the table at some point as B. The marble’s inertia of motion 
and the friction between the marble and the table have caused the 
marble to follow the curved path. The deflection has been con¬ 
tinuously to the right of the straight course CA as the marble moved 
into regions of greater rotational velocity. 

If, instead, the marble be started from A along AC at the instant 
the table is set rotating, in one second it will have traversed the 
path A'B'. The inertia of the marble has again made it take a 
curved path, deflected to the right of a straight course, as it moved 
into regions of less rotational velocity. 

On any straight line that may be drawn upon the table, along 
which the marble may be started, the marble will either approach 
the center or recede from it; and since in both cases the counter¬ 
clockwise direction of rotation causes a right-handed deflection, the 
marble will in all cases be deflected to the right of the straight course 
along which it is started. 

The rotation of the earth, as seen from above the north pole, is 
counterclockwise; and winds in the northern hemisphere behave 
in a similar way to that of the marble described above in I. 

As seen from above the south pole the earth’s rotation is clock¬ 
wise, and deflection is to the left of a straight course, as shown in II. 

The deflective effect of rotation may be illustrated by pouring 
water on a rotating globe. If the globe be held with its axis verti¬ 
cal, and rotated in a direction corresponding to that of the earth, 
the minute streams will be deflected to the right of the meridians 
along which they start so long as they are in the northern hemisphere 
and to the left after they cross the equator. If started from the 
south pole of the globe, they will suffer left-handed deflection first, 
changing to right-handed upon crossing the equator into the north¬ 
ern hemisphere. 

The only winds which do not suffer deflection are those along the 
equator. Set moving in any other direction or in any other latitude, 
they suffer deflection according to Ferrel’s Law. The deflective 
effect of rotation increases with the latitude, and in any latitude 
varies inversely as the velocity of the wind. It should be remem- 


124 


NEW PHYSIOGRAPHY 


bered that the rotation of the earth has no power to set the air in 
motion, and its deflective power exists only after motion is started. 

Two important laws governing winds are: 

1. Winds always blow from a region of higher pressure to a 
region of lower pressure, with a velocity which varies with the 
pressure gradient. 

2. On account of the rotation of the earth winds turn to the 
right of the pressure gradient in the northern hemisphere and 
to the left of the pressure gradient in the southern hemisphere. 

Description of Wind Belts. — The wind belts depend upon 
the pressure belts, and these in turn are determined by the 
distribution of temperature. Since the temperature shifts, 
in like manner the pressure and wind belts shift. 

The doldrums are so named because of the light winds and 
calms that characterize this belt. It is a belt of high tem¬ 
perature, and consequently low pressure. The winds move 
obliquely in toward the doldrums from both sides. The low 
pressure is of convectional origin, and the movement of the 
air in this belt is chiefly upward; hence it is a belt of calms. 

The trades are the winds that blow from either side 
obliquelylh toward the doldrum belt. They derive their 
name from their constancy of duration and direction, quali¬ 
ties important to sailing vessels engaged in trade. In the 
northern hemisphere they are called northeast trades, and in 
the southern, southeast trades. They have their origin in the 
high-pressure belts of the horse latitudes, and move toward 
the low-pressure belt of the doldrums. Instead of following 
the steepest gradients along the meridians, as they would 
do if there were no rotation, they are deflected, in accordance 
with Ferrel’s Law, to the right in the northern hemisphere, 
and to the left in the southern. 

In order to reach the doldrum belt the trade winds some¬ 
times have to cross the equator. When northeast trades 
cross to the south of the equator, they become northwest 


MOVEMENTS OF THE AIR 


125 




Fig. 58. — Terrestrial Winds for January 










126 


NEW PHYSIOGRAPHY 


winds; whereas southeast trades crossing the equator be¬ 
come southwest winds. These winds, sometimes called 
hooked trades, are due to the difference in the deflective 
influence of the earth’s rotation north and south of the 
equator. 

The horse latitudes are belts of high pressure next to and 
poleward from the trades. The air that is warmed, and by 
convection rises in the doldrums, moves away poleward at 
high altitudes. Its overflow aloft, and consequent piling 
up in the regions of the horse latitudes, causes the high-pres¬ 
sure and descending currents in these belts. As a result of 
its descent, the air is warmed by compression. These are 
belts of light winds and calms. The trade winds begin here; 
likewise the less regular winds that move away toward the 
poles. 

The prevailing westerlies flow away from the horse-latitude 
belts toward the poles, being deflected to the east by the 
rotation of the earth. They are named from their direction, 
and in their upper parts are continuations of the overflow 
above the trades. The prevailing westerlies are neither so 
constant in duration nor in direction as the trades. 

As the prevailing westerlies approach the poles obliquely 
and along converging courses, in each hemisphere a circum¬ 
polar whirl develops. These winds spiral about the north 
polar region in a left-handed or counterclockwise direction, 
and about the south polar region in a right-handed or clock¬ 
wise direction. 

As a result of the spiral inflow toward the poles, it is be¬ 
lieved the polar regions are areas of low pressure. The centrif¬ 
ugal force resulting from the whirl tends to heap the air 
out around the polar center and to produce a circumpolar 
ring of high pressure. From this ring the winds move away 
into lower latitudes. The frequent northeast winds observed 
on the northern coast of Alaska are thus explained. 

Both temperature gradient and pressure gradient between 
the equator and the polar regions are steeper in winter than 


MOVEMENTS OF THE AIR 


127 


in summer. As a consequence the circumpolar whirls are 
strongest in winter. 

Because of the excessive cooling of the northern continents 
in winter, the North Atlantic and North Pacific oceans are 
warmer than the northern continents and are therefore cen¬ 
ters of low pressure. About these centers the winds spiral in 
a way comparable to the circumpolar whirl, and from these 



p IG 59 # — Winds of Northern Indian Ocean for January 


secondary centers winter cyclones are projected. Those from 
the Pacific often move southeastward into Canada and the 
United States. 

Shifting of the Wind Belts. — The pressure belts and wind 
belts follow the shifting of the belt of greatest heat. As a 
result of this shifting, places near the border of the various 
wind belts fie alternately in two belts. Southern Florida, 
southern California, and northern Mexico fie alternately 
under the northeast trades and the horse latitudes; and the 






































128 NEW PHYSIOGRAPHY 

Panama Canal Zone, in winter under the northeast trades, 
is in summer in the doldrum belt. The Amazon Valley, for 
the most part in the doldrums, is periodically in part swept 
over by the trades. 

Monsoon Winds. — Places so situated as to have seasonal 
change of winds are said to have monsoon winds, or simply 



monsoons . While many regions have monsoon winds, per¬ 
haps the most pronounced and best-known monsoons are 
those of the northern Indian Ocean and the adjacent lands 
to the north and east. 

During the northern summer the heat equator migrates 
far into the heated continent of Asia. Then the southeast 
trades, which run to the doldrum belt, cross the geographical 
equator and in the northern hemisphere become, according 
to Ferrers Law, southwest winds. In winter the heat equator 





















































MOVEMENTS OF THE AIR 


129 


migrates southward, and over southern Asia and the northern 
Indian Ocean the strengthened northeast trades blow. 

Continental Air Drifts. — The land has a higher tempera¬ 
ture than the sea in the same latitude in summer, and a 
lower temperature in winter. As a result the continents are 
areas of low pressure in summer and of high pressure in winter. 
Following this arrangement of pressure there is a general 
outward drift of the lower air from the continents in winter, 
and an inward drift toward the continents in summer. 

These movements, which in a strict sense are monsoons, are 
never so called, but are sometimes called continental winds. 
They are so easily obscured by other winds as to be scarcely 
noticeable of themselves, and their chief office is to modify 
other winds. Thus upon our western coast the westerlies are 
weakened in winter and strengthened in summer, whereas 
upon our eastern coast they are strengthened in winter and 
weakened in summer. 

Land and Sea Breezes. — The winds that blow alternately 
from and to the land, at and near the seashore, are called 
land and sea breezes. When the land is colder than the sea, 
the breeze blows from the land, and is called the land breeze; 
and when the land is warmer than the sea, the breeze blows 
from the sea, and is called sea breeze. The land breeze blows 
during the night, as the land is then an area of relatively 
high pressure; and the sea breeze blows during the day, the 
land being then a region of relatively low pressure. 

Advantage is taken of this twice daily change of breeze 
by fishing and pleasure craft that depend upon sails to carry 
their fleets away from and to bring them back to land. 
Similar land and lake breezes are felt along the shores of our 
great lakes and inland seas. 

If the land should remain colder than the sea through¬ 
out the twenty-four hours, then there would be a continuous 
land breeze. This often happens in winter, especially when 
the land is covered with snow. On the other hand, in mid¬ 
summer it sometimes happens that the land does not cool 


130 


NEW PHYSIOGRAPHY 


down during the night below the temperature of the adjacent 
sea; then the sea breeze continues throughout the night. 

Mountain and Valley Breezes. — Another similar change 
of breeze may be noted upon the slopes of mountains or of 
deep valleys. The top of the mountain, or of the dividing 
ridge, being first warmed by the rays of the rising sun, con- 
vectional ascent begins there. As the slopes and the air in 
contact with them are warmed to lower and lower levels, the 
warmed lighter air finds an easier escape obliquely up the slope 
and through the convectional chimney above the summit 
than vertically through the overlying stratum of inert air. 
This obliquely ascending current is felt upon the slope as a 
valley breeze. 

At night radiation is most rapid from the mountain sum¬ 
mit or ridge crest; and the cooled air from the upper slopes, 
being heavier, moves down the slopes toward the valley. 
This is the mountain breeze. The alternation of mountain 
and valley breezes may be interrupted in the same way as 
described for land and sea breezes. 

Avalanche Winds. — These winds occur in valleys between 
steep mountain slopes. They are caused by the moving 
mass of snow and rock on the mountain side. Being com¬ 
pressed in the narrow valleys, these winds sweep with 
almost irresistible force down the valley. 

Cyclonic Winds. — In temperate latitudes, by far the most 
important winds are those unperiodic winds, caused by the 
irregular distribution of pressure in lows and highs, and known 
as cyclonic winds. These lows and highs are best understood 
if considered as local disturbances in the terrestrial winds. 
Thus considered, their general drift with the winds of the 
belt in which they occur is accounted for. 

Cyclonic winds move obliquely in toward the center of a 
low, spiraling about the center in a counterclockwise direction 
in the northern hemisphere and in a clockwise direction in 
the southern hemisphere. They move spirally out from a 
high, turning in a clockwise direction in the northern 


MOVEMENTS OF THE AIR 


131 


hemisphere and in a counterclockwise direction in the 
southern. 

Although their origin is obscurely understood, some lows 
are known to be of convectional origin; others are probably 
not. This has given rise to two classes of cyclones, convec¬ 
tional and nonconvectional. 

Land varies indefinitely in its power to absorb insolation, 
owing to its almost infinite variety of composition, covering. 



Fig. 61. — Low in Northern Hemisphere 

Winds deflected to the right of the gradient. In the southern hemisphere 
deflection is to the left. 


and form. If any given limited area is heated in excess of the 
surrounding region, the air above this area expands and 
overflows aloft. The area then becomes a low , while the 
surrounding region has increased pressure. The overflow 
from adjacent lows may unite and produce a high. On the 
other hand, the air over a given area, such as a snow-covered 
region, may be excessively cooled and condensed. Then 
the upper air from the surrounding region flows in upon it, 
producing a high , surrounded by a ring of lower pressure. 
Such lows and highs are of convectional origin. 

It seems probable that nonconvectional cyclones originate 





132 


NEW PHYSIOGRAPHY 


in two ways. In high latitudes about the margins of the 
circumpolar whirls and in the northern hemisphere about 
the margins of the North Atlantic and North Pacific eddies, 
smaller eddies of air are developed — eddies similar to the 
eddies that arise about the edge of a strong water whirl. 
These are sometimes called driven cyclones. They are most 
frequent and are best developed in winter when these great 
eddies are strongest. In lower latitudes, even within the 



Winds deflected to the right of the gradient. In the southern hemisphere 
deflection is to the left. 

tropics, cyclonic whirls may result from the friction between 
great poleward moving and equatorward moving masses of 
air. Such cyclones are called frictional cyclones. They may 
originate in the higher currents, and sink to the bottom of 
the air fully developed. 

Movements of Lows. — Four distinct movements with 
reference to lows must be noted: the oblique movement of 
the winds toward the low center, their ascent as the center is 
approached, spiral outflow above, and the forward movement 
of the low itself. 







MOVEMENTS OF THE AIR 


133 


Wherever their place of origin, most lows finish their course 
in the zone of westerlies, following the general direction of 
these winds. In the United States three general paths are 
followed, as the cyclone originates in the northwest, in the 
southwest, or in the southeast within the tropics. Those 
originating in the northwest move first southeasterly, until 
they reach the axis of the Mississippi Valley, when they 
change to a northeasterly course for the remainder of their 
journey across the continent. Those having their origin in 
the southwest move systematically northeastward across 
the continent. Tropical cyclones having their origin in the 
region of the West Indies move first northwestward until 
they reach the high-pressure calms, north of which they 
conform to the course of the westerlies. These usually cross 
the high-pressure belt over or near the land, the belt being 
less well-developed there. 

All cyclones sooner or later conform to the course of the 
westerlies. The upper air currents in all latitudes move to 
the northeast in the northern hemisphere and to the south¬ 
east in the southern; and it is probable that the spirally 
ascending column of air at the cyclonic center reaches to 
these upper currents. 

Strength of Cyclonic Winds. — The velocity of the wind 
increases as it approaches the center of a low; and if a strong 
spiral movement is developed, the velocity is greatly in¬ 
creased. On the other hand, the winds start from the center 
of the high and are therefore weakest there. Consequently, 
the strength of the wind increases with the approach of a 
low and decreases with the approach of a high. 

Because of their greater strength, cyclonic winds obscure 
all other winds in regions where they occur. The winds 
poleward from the horse latitudes are chiefly cyclonic. 

When the low-pressure area is very small, the spiraling 
winds may attain excessive velocity, and become destructive. 
Such wind storms are known as tornadoes. In their greatest 
strength the winds are so strong as to carry away the in- 


134 


NEW PHYSIOGRAPHY 


struments for their measurement. Velocities of more than 
100 miles an hour have been measured. Similar but larger 
storms in the West Indies are called hurricanes, and in the 
East Indies typhoons. 

The term cyclone is often used by the uninformed to refer 
to especially strong winds, but in reality a cyclone may be a 
gentle breeze as well as a driving gale. The strength has noth¬ 
ing to do with its being a cyclone. A cyclone is simply a low- 



pressure area; and the winds circulating around such low 
pressure, often of 500-1000 miles in diameter, may have 
any velocity. Hurricanes and typhoons are winds of unusual 
violence, sometimes attaining velocities of more than 60 miles 
an hour, with diameters about 100 miles or more. Tornadoes 
are very violent storms which sometimes have wind of more 
than 100 miles an hour but with diameters often less than 
100 feet. It seems as if the more limited the area of storm, 
the stronger the wind, for the steeper pressure gradients 
usually accompanying more limited areas. 

Cyclones of wide extent, within the tropics, sometimes have 











MOVEMENTS OF THE AIR 


135 


within the whirl of destructive winds an area of clear skies. 
This area, called the eye of the storm, may be as much 
as one-tenth of the diameter of the cyclonic area. Vessels 
passing through the eye of the storm experience equally 
strong winds in the front and in the rear of the cyclone, 
though from opposite directions. 

Shifting of Winds. — When a station lies near the path 
of a low or high, the winds at the station shift in a system¬ 



atic way as the barometric disturbance passes. In the 
westerlies of the northern hemisphere, where the disturbances 
advance from west to east: 

1. If a low passes north of the station, the first effect is to induce 
easterly winds. These veer (change with the sun) through the south¬ 
east, south, and southwest to some westerly quarter as the dis¬ 
turbance advances easterly. 

2. If a low passes south of the station, the winds, northeasterly at 
first, back (change against the sun) through the north, northwest, 
and west to a southwesterly direction. 

3. If a high passes north of the station, the winds, at first westerly 



136 


NEW PHYSIOGRAPHY 



Fig. 65 

The path of the center AB is here south of the station S. As above, the 
curved arrow XY is the composite of the sheaf of arrows through S. 



A high passes north of the station S; and the succession of winds, blowing out from 
the center of the high, is shown by the sheaf of arrows through S. The curved arrow 
X Y shows their composite. 


or northwesterly, veer to northerly and northeasterly winds in 
succession. 

4. If a high passes south of the station, the wind backs from the 
southwest through the south, southeast, and east in turn. 







MOVEMENTS OF THE AIR 


137 


Should the disturbance pass centrally over the station, the 
wind will hold steadily as a northeasterly wind if the dis¬ 
turbance is a low, or as a westerly wind if the disturbance 
is a high, as the disturbance advances, changing suddenly 
through a calm to the opposite point of the compass as the 
center of the disturbance passes. 

In any other wind belt the direction of shifting may be 
determined by noting the direction in which the disturbance 



advances and considering the direction of movement of 
winds about highs and lows in that belt. For any given sta¬ 
tion the direction of shifting is systematic and uniform for a 
given set of conditions. 

Special Winds. — In every part of the world winds of 
special character and of exceptional occurrence are known, 
and local names are given to them. Some of these are warm 
winds, others are cold; some owe their character to change 
in latitude , others owe theirs to a change in altitude . 

The change in temperature due to change in latitude is, 
on an average, about 1° Fahrenheit for each degree of lati- 




138 


NEW PHYSIOGRAPHY 


tude, whereas the temperature change in ascending currents 
of air is about 1.8° F. for 300 feet. Therefore, any transfer of 
air, either in latitude or altitude, is accompanied by a change 
in temperature, both of the transferred air and of the region 
to which it is transferred. 

Generally speaking, winds blowing from lower to higher 
latitudes are warm winds, while those blowing from higher 
to lower latitudes are cool. Winds descending from 
higher to lower altitudes are generally cool or cold winds, 
though they become warmer as they descend. Those blow¬ 
ing up a slope are likely to be warm. 

Among warm winds may be mentioned: 

The hot wave, western central United States. It blows in summer 
from the west or southwest, sometimes continuing for days and 
withering all growing vegetation. 

The sirocco, a south wind from the Sahara Desert, felt as far as 
the north shore of the Mediterranean Sea. It is commonly dry and 
dust laden. 

The simoom, an intensely hot, dry, and generally sand-laden 
wind of the Arabian Desert. It is probably a convectional whirl¬ 
wind, similar to the dust-laden whirlwinds of all dry, hot climates. 
It lasts usually less than ten minutes, and often forms sand spouts. 

The chinook, an American wind, which moves down the slopes 
of mountains toward a low-pressure area at their base. Though 
starting upon its descent as a cold wind, it warms by compression 
in its descent; and if the mountain is high, it may reach the base 
as a warm or even hot wind. In all cases, because of its dryness, it 
evaporates or melts the snow fields over which it blows and often 
causes destructive avalanches by melting the snow on the steep 
slopes. It is Of frequent occurrence along the eastern bases of the 
Rocky and Sierra Nevada Mountains. Many valleys here are kept 
practically free from snow; and their temperatures are so mild as 
to make shelter for stock in winter unnecessary and to permit graz¬ 
ing throughout the year. The chinook is scarcely noticeable in 
summer. 

The foehn is the European wind of the chinook type, common on 
the northern slopes of the Alps, where, in the north-south valleys 


MOVEMENTS OF THE AIR 


139 


it hastens the ripening of grapes in the fall and in winter rapidly 
melts the snows in its path. This has earned for it the name of 
“ snow-eater.” 

To the class of cold winds belong: 

The norther of the southwestern United States. It is the cold 
inflow of winds from the north at the rear of the winter cyclone. 
A fall of temperature of 50° in two hours has been noted. 

These winds often cause great suffering, loss of live stock, and 
even loss of human life. 

The blizzard, the American name for a cold wind of high velocity, 
accompanied by snow. It is common east of the Rocky Mountains 
and often causes great loss of life among stock. 

The bora and mistral are European cold winds. 

Velocity of Winds. — Winds are sometimes classified 
according to their velocity. The velocity of the wind is 
measured by means of the anemo¬ 
meter, which is gauged in miles per 
hour or feet per second. 

When the wind reaches a velocity 
of 100 miles or more per hour, the 
instruments are usually carried 
away. Therefore, we have no 
record of the velocity of the wind 
in our most violent tornadoes. 

The accompanying table gives 
the ordinary names for winds, 
their approximate velocities in 
miles per hour, and a common and 
easy way of judging them: 

Name Distinctive Characters Velocity 

Calm.Flags limp, leaves unmoved. 0 

Light breeze.Moves leaves of trees. 1-5 

Fresh wind.Moves branches of trees, blows up dust... 5-15 

Brisk to strong . . . Sways branches of trees, makes whitecaps. . 15-25 

High wind.Sways trees, moves twigs on ground.25-35 

Gale ..Breaks branches, dangerous sailing.35-75 

Hurricane wind .. .Destroys houses, uproots trees.75—100 













140 


NEW PHYSIOGRAPHY 


QUESTIONS 

1. Why does the wind blow? Why is there not always wind? 

2. What determines the direction and volocity of the wind? 

3. Interpret “no wind” in terms of pressure gradient. 

4. Why are summer days, as a rule, more apt to be windy than 
summer nights? Than winter days? 

5. Why are the “trades,” on the whole, stronger than the “wes¬ 
terlies”? 

6. Why are upper currents stronger than surface winds, and 
winter winds stronger than summer? 

7. Why are the “ trades” more regular over the sea than over the 
land? 

8. Why are the upper currents in all latitudes westerly currents? 

9. Why do winds spiral about “lows”? Why are not equally 
strong whirls developed about “highs”? 

10. Why are southern California and southern Florida more 
likely to have westerly winds in winter than in summer? 

11. Why are storms that come from the southwest often called 
northeasters f 

12. Why do cyclones in the eastern United States move so gen¬ 
erally toward the northeast? 

13. Why do thunderstorms occur so rarely at night? 

14. What should be the direction of shifting of the wind with 
passing “lows” and “highs” at Havana, Cuba? At Buenos Aires? 

15. Why, in the natural ventilation of our rooms, do we admit 
the air at the bottom of the room, whereas in forced ventilation the 
air is admitted at the top? Which is better? 

16. Why is a room better ventilated when windows are both 
raised from the bottom and lowered from the top than when merely 
raised or lowered? 

17. Why do fires burn better in cold than in warm weather? 

18. Why is a flue less likely to smoke after the fire is well started 
than when it is first lighted? 

19. Why do snowdrifts and sand dunes accumulate behind an 
obstacle rather than in front of it? 

20. Why do aviators avoid flights over cities and deep canyons? 


CHAPTER X 


MOISTURE OF THE AIR 

How Moisture Enters the Air. — Most of the moisture of 
the air is supplied by evaporation from moist surfaces. 
Evaporation is the process hy which solids and liquids are 
changed to vapor. The molecules of the water are thought to 
be in rapid motion, moving in all directions. Many of those 
moving toward the surface pass beyond the surface into the 
air. At the same time the molecules of water in the air, 
moving toward the water surface, reenter the water. If 
those passing out exceed those returning to the water, 
evaporation is said to be going on; if the number of molecules 
leaving and returning to the water are equal, evaporation is 
said to have stopped, and the air is said to be saturated. 

While evaporation is chiefly from water surfaces, yet 
scarcely any land surface is so dry that it does not supply 
some moisture to the air. The lower air is never wholly free 
from moisture, the amount present depending chiefly upon 
the temperature of the air. Evaporation takes place at all 
temperatures. The moisture of the air is for the most part 
in its invisible form, that of water vapor; but it becomes 
visible when, by cooling, the vapor changes to the liquid or 
solid state as clouds. Real steam issuing from a boiling kettle 
is invisible. So-called steam, the visible cloud seen a few 
inches away from the spout, is composed of minute particles 
of water, and not water vapor. 

Humidity of the Air. — The air at all temperatures has 
a certain capacity for water vapor. Other conditions re¬ 
maining the same, the higher the temperature the greater 
the capacity. To satisfy this “ thirst ” of the air, evaporation 
goes on. If the temperature is low, or the capacity of the 

141 


142 


NEW PHYSIOGRAPHY 


air is nearly satisfied, evaporation is slow; and when the 
capacity is fully satisfied, evaporation ceases. The air is 
then said to be saturated. Saturation may result from (1) 
continued evaporation without change of temperature or (2) 
a lowering of the temperature. 

The condition of the air as regards the amount of water 
vapor present is called its humidity. The actual amount of 
water vapor in a given volume of air, as the number of grains 
of water vapor in a cubic foot of air, is its absolute humidity. 
The amount of water vapor present, divided by the amount 
the air is capable of holding at the time, is its relative hu¬ 
midity. Relative humidity is expressed in percentages. 

Dew Point. — If the air at a given temperature has six 
grains of water vapor present to the cubic foot of air and 
its capacity for water vapor is 10 grains to the cubic foot, its 
absolute humidity is “ six grains per cubic foot,” and its rela¬ 
tive humidity is 60 per cent. When the air is saturated, 
its relative humidity is 100 per cent. The temperature of 
saturation is called the dew point. 

Condensation. —- As evaporation is promoted by increase 
of temperature, so it is retarded by any lowering of tem¬ 
perature; and when the air is saturated evaporation stops, 
or if it continues, condensation goes on at an equal rate. 

When the air is saturated, any further lowering of the 
temperature causes the water vapor in the air to change to 
the liquid or solid state, depending upon the temperature 
at which the change takes place. This change is called 
condensation. It is condensation not of the air, but of the 
water vapor of the air. It should be remembered that con¬ 
densation can occur only as a result of the cooling of the air. 

The cooling which causes condensation may result from: 

1. Loss of heat by radiation, chiefly to the land or water 
from the lower air, when fogs are likely to result. 

2. Contact with cold surfaces, when dew or frost may form. 

3. Horizontal mixing of cold and warm currents, when 
clouds, fogs, or precipitation may occur. 


MOISTURE OF THE AIR 


143 


4. Mechanical cooling by expansion , incident to ascending 
currents, producing clouds or yielding precipitation. 

'jpr The last is probably the most effective cause of condensa¬ 
tion. Air in rising expands and is mechanically cooled at 
the rate of about 1.8° F. for every 300 feet of ascent. This 
rate is the same whether the air rises by convection or is forced 
to ascend by reason of winds blowing against a rising land 
surface. 

When air descends, as at the center of a high, or on the 
leeward slope of mountains, it warms up at the same rate. 

The doldrum belt, the regions of lows, and the windward 
slopes of mountains are, therefore, apt to be rainy, while the 
high-pressure calm belts, the regions of highs, and the leeward 
slopes of mountains have prevailingly clear skies. 

If saturation is reached at a temperature at or lower than 
freezing, with further cooling the water vapor changes at 
once to the solid state, without passing through the liquid 
state; but if saturation occurs above the freezing point, the 
condensed product is liquid. As water vapor may change 
immediately to the solid state, so ice may evaporate without 
melting. 

It is a familiar fact that snow gradually disappears from 
the land during days when the temperature does not rise to 
the freezing point, and a wet garment hung in air of freezing 
temperature becomes dry. Roads in winter become dusty 
though continuously frozen. 

Effects upon Temperature. — In order to change ice or 
water to vapor, heat is absorbed or becomes latent. This 
heat is used in the mechanical task of driving the molecules 
of ice or water apart and is lost as far as affecting tem¬ 
perature is concerned. The vapor formed is of the same 
temperature as the substance from which it came. The 
heat necessary for evaporation is obtained from surround¬ 
ing objects, chiefly from the lower air, so that evaporation 
has the effect of cooling the air. Evaporation is always a 
cooling process with respect to surrounding objects. 


144 


NEW PHYSIOGRAPHY 


The air in the upper part of a cloud layer is distinctly 
cooler than that above the cloud, owing to cooling by evapo¬ 
ration of the cloud particles. 

The custom of fanning to cool one’s self is an illustration 
of cooling by evaporation, the constantly changing air in 
contact with the moist skin taking up the moisture more 


rapidly. 

A thermometer suspended in front of a rapidly rotating 
fan registers a slightly increased temperature, there being 
no evaporation from its surface, 
and the air being slightly com¬ 
pressed against the thermometer. 
If, however, the thermometer bulb 
be wrapped in a thin piece of moist 
muslin, the draft from the fan 
causes a marked lowering of the 
thermometer. 

Gasoline, bay rum, or any 
easily vaporized liquid, when 
rubbed upon the hands, cools 
them; and the high fever tempera¬ 
tures may be cooled by alcohol 
baths. The flesh may be frozen 
by the applicaion of ether. 

To Determine the Humidity of 
the Air. — The principle of cool¬ 
ing by evaporation is used in the 
construction of the psychrometer, 
an instrument for the determina¬ 
tion of the humidity of the air. 
It consists simply of a wet-bulb and a dry-bulb thermo¬ 
meter. When the bulb of one thermometer is kept con¬ 
stantly moist, evaporation from the moist surface cools the 
mercury in the enclosed bulb. The wet-bulb thermometer, 
therefore, commonly shows a lower temperature than the 
dry bulb. The more rapid the evaporation the greater the 






















































































MOISTURE OF THE AIR 


145 


difference in the readings of the two thermometers, and the 
drier the air. Little difference in the readings indicates slow 
evaporation and a humid air. When evaporation ceases, as 
when the air is saturated, the two thermometers show the 
same readings. To get a correct reading of the wet-bulb 
thermometer, the bulb should be fanned to replace the 
humid air next the bulb by the drier air farther away. 

Temperature Due to Condensation.— When condensation 
takes place and vapor is changed to liquid or solid, the latent 
heat of evaporation is released; and the heat thus liberated 
becomes available for affecting temperature. Surrounding 
objects are then warmed. Condensation of water vapor is, 
therefore, a warming process. In the layer of air where 
clouds are forming and from which rain and snow fall, a vast 
amount of heat is liberated by the condensation of the water 
vapor , thus tending to check further condensation by warming 
the air. 

A burn from steam at 212° F. is much more severe than a 
burn from water at the same temperature, since a great 
amount of. heat is liberated in reducing steam to a liquid 
without changing its temperature. 

Distribution of Water Vapor. — Since most evaporation 
takes place at the bottom of the air, it follows that the lower 
air is of higher absolute humidity than the air at greater 
altitudes. Also the air over the oceans and other water sur¬ 
faces is more humid than the air over the land. But by 
diffusion and by vertical currents the water vapor is distrib¬ 
uted generally through the lower air to a height of about 
six miles. Water vapor, and therefore clouds, are practi¬ 
cally confined to the lower six miles of the air. 

Although there is more water vapor in the lower air, the 
relative humidity of the lower air is not usually so high as that 
at greater altitudes, because of the lowering of temperature 
with increase of altitude. Clouds and rain are usually the 
result of condensation in the upper air, since the air cools 
in rising, because of expansion. This is perhaps the most 


146 


NEW PHYSIOGRAPHY 


important, though not the only, cause of condensation of 
water vapor. 

The relative humidity of the air varies not only with 
change of place or of altitude, but also from hour to hour at 
any place. It is highest at the coolest hour of the day, 
usually in the early morning, and decreases as the day ad¬ 
vances and the air warms up. It is lowest at the warmest 
hour of the day, usually from one to three o’clock p.m., after 
which hour it slowly rises. 

Dew and Frost. — When air at temperatures above the 
dew point is cooled to saturation by contact with objects 
below the dew point, condensation upon the cooling object 
occurs. If saturation occurs above freezing, the condensation 
is liquid, and we call it dew. If saturation occurs below 
freezing, the vapor changes at once to ice without visible 
liquefaction, and this is called frost. Dew and frost form; 
they do not fall. Frost is not frozen dew. Frozen dew becomes 
transparent pellets of ice. 

These forms of condensation are most common on or near 
the surface of the ground. At a height of five feet there may 
be no frost or dew formed on plants when the ground and ob¬ 
jects near it, as the grass, are white with frost or wet with dew. 

Anything that checks the cooling of the ground and lower 
air tends to prevent the formation of dew and frost. The 
ground beneath trees and shrubs is often protected from 
dew and frost when these form freely upon the unprotected 
ground. A cloudy sky, by checking radiation, prevents 
excessive cooling and hinders the formation of dew or frost. 
Likewise wind, by constantly changing the layer of air next 
to cold surfaces, hinders cooling and condensation. It is 
rare to have frost or dew on cloudy or windy nights. 

It is a commonly recognized fact among dwellers in the 
country that clearing skies and the “ laying ” of the wind 
toward nightfall may bring frost at those seasons when 
early planted or late maturing crops would be injured by it. 

Many orchardists protect their trees from frost by building 


MOISTURE OF THE AIR 


147 


smudges among the trees. The smoke cloud thus produced 
hangs above the orchard, checks radiation as a blanket 
would, and thus often prevents frost. 

When condensation begins, the liberation of latent heat 
tends to check further cooling; so if saturation occurs much 



Fig. 70 — Valley Fog 


above freezing, frost is unlikely, because freezing temperatures 
are not likely to be reached. 

Housewives often protect their flowers from frost on cold 
nights by exposing shallow pans of water in the warm room 
with the plants, thus greatly increasing the humidity. When 
the room cools saturation will be reached at a temperature 
well above freezing; and the liberated latent heat, as con¬ 
densation goes on, may suffice to keep the temperature above 
freezing. 

This principle has been applied on a large scale for the 
protection of orchards. By spraying water into the air in 
and about the orchard, the farmer increases the humidity and 
raises the dew point. 



148 


NEW PHYSIOGRAPHY 



Fig. 71. — Cirrus Cloud 


Clouds. — When the vapor in the air is condensed above 
the surface of the ground into particles of water or ice so 
small that they remain suspended in the air, the product of 
condensation is called cloud. 

If the cloud is so low that it reaches the land or water, it 
is called fog. 

Clouds are classified chiefly by their form. The thin, 
feathery-white clouds, seen high in the air, and frequently on 
fair days, are cirruJ clouds. These filmy clouds are, for the 
most part, composed of slender crystals of ice called spicules, 
and are the highest clouds. Their average summer altitude 
is about six miles, and they generally move eastward at the 
rate of about 60 miles an hour. In winter their average 
height is about five miles, and their eastward motion is 
about 100 miles an hour. 

Cirrus clouds are often forerunners of storms, being the 
high-level overflow in front of the cyclone center. 

Cumulus clouds are the massive piles of cloud, with an 



MOISTURE OF THE AIR 


149 


even base, and resembling piled-up fleeces of wool or volumes 
of condensing steam and smoke from a locomotive. They 
are the result of rising currents of air, usually of convectional 
origin, and are therefore storm clouds. These clouds, some¬ 
times called thunderheads , are land clouds rather than sea 
clouds, day rather than night clouds, and are more com¬ 
monly in motion than at rest. 

The average summer height of cumulus clouds exceeds a 
mile, whereas their winter height is somewhat less; and 
their summer and winter velocities are about six and nine 
miles an hour respectively. Their even bases are usually 
from one-fourth to one-half mile above the surface. 

Nimbus is the name given to any cloud from which rain 
or snow falls or may be expected to fall. These clouds are 
of very variable height, being, on the whole, higher in 
summer than in winter. Their height decreases toward the 
poles. 



Fig. 72. — Cumulus Cloud 





150 


NEW PHYSIOGRAPHY 


Nimbus clouds are of an even grayish tint, sometimes 
completely overcasting the skies for hours and even for 
days continuously. 

Stratus is any cloud spread out in parallel sheets or bands. 
It is a night rather than a day cloud, and more common over 
the sea and in valleys than over the higher lands. It in¬ 
cludes fogs. The bands of clouds seen at higher levels may 
be designated by such compound names as cirro-stratus and 
cumulo-stratus. 

Fog is the cloudy condensation near, or resting on, the 
land or sea. It usually results from warm, humid air passing 
over cold surfaces. In winter, winds blowing from the sea 
upon land are likely to produce fogs. An east wind upon 
eastern coasts and a west wind upon western coasts are the 
chief sources of fogs. 

Fogs are more frequent in valleys than on the slopes or 
tops of hills and mountains. The cooler, heavier air accu¬ 
mulates in the valley, where there is likewise more moisture; 
and these two conditions combine to produce the fogs in 
valleys and lowlands. 

The fogs of Newfoundland are known to all navigators of 
the North Atlantic. The warm and cold ocean currents or 
drifts which meet there are accountable for the fogs. When 
the winds are from an easterly quarter their temperatures 
are lowered and their moisture condensed as they pass over 
the cold Labrador current to the west. Another cause of 
much of the Newfoundland fog is the ice brought down from 
the Arctic regions by the Labrador current. These icebergs 
are apt to be centers of dense fogs, especially in summer. 
Fogs off the Grand Banks seriously delay and endanger 
ships passing through them, requiring reduced speed and 
often complete stops with continuous warning signals. 
They are also the greatest obstacle to trans-Atlantic 
travel by airplane or airship. Traffic in many of our 
harbors is frequently interrupted by the dense fogs that 
envelop them. The fogginess of London and manufactur- 


MOISTURE OF THE AIR 


151 


ing cities is greatly increased by reason of the particles of 
soot issuing from chimneys. Each soot particle is a center 
of cooling and condensation. These fogs in London are some 
times so dense as to stop traffic. They are there known 
as “ pea-soup ” fogs. 

Clouds serve as blankets to equalize temperatures. 
Summer days are cooler, and winter days and nights are 
warmer when the sky is overcast. 



Fig. 73. — Snow Crystals 
Photo by United States Weather Bureau. 


Rain and Snow. — When condensation takes place in the 
air, and the condensed particles become heavy enough to 
sink through the air, the process is called 'precipitation. If 
saturation occurs above freezing, the product is rain; if 
below freezing, snow. As in the production of frost, so in 
snow, the water vapor passes at once from the vapor condi¬ 
tion to the solid. Rain and snow bear the same relation to 
each other as do dew and frost. Snow is not frozen rain. 

Although clouds almost invariably precede rain and snow, 




152 


NEW PHYSIOGRAPHY 


precipitation without clouds may occur. Such cloudless rain, 
usually in very fine drops and more resembling mist, is 
called serein. 

Snowflakes are crystallized water vapor , built up on pat¬ 
terns resembling beautiful six-rayed stars. The size of the 
flake as well as the exact pattern seems to depend upon the 
temperature at which the flake is formed. 

In all latitudes an altitude may be reached at and above 
which the precipitation, even in summer, is chiefly in the 
form of snow. At a slightly lower level more snow falls than 
melts during the year, with the result that there is perennial 
snow. The lower limit of perennial snow is known as the 
snow line. It is about 16,000 feet above the sea at the equa¬ 
tor and descends gradually toward the poles, reaching sea 
level within the polar circles. 

Sleet and Hail. — If raindrops in their fall pass through 
increasingly colder air and are frozen into small pellets of ice, 
the product is sleet. As this arrangement of temperature is 
most likely to occur in winter, when the land is colder than 
the lower air above it, sleet is a winter phenomenon. Sleet 
is frozen rain. 

In summer, especially on hot afternoons, and near the 
center of a cyclonic storm, large pellets of ice called hail 
often fall. Upon examination hailstones prove to be made of 
concentric layers of ice. This structure, together with their 
often great size, suggests that they ar e L froze n raindrops 
enlarged by successive condensations and freezings upon 
their surfaces. Their often spongy texture indicates that 
snow may also enter into their composition. Hail is chiefly a 
summer phenomenon. 

Hailstorms are sometimes very destructive. Their paths 
are usually only a few miles in width, and fortunately not of 
great length; for often growing crops, orchards, and even 
forests are destroyed. Leaves, bark, and branches are 
stripped from trees; young animals are killed, and windows 
and roofs broken by the hailstones. 



MOISTURE OF THE AIR 153 

It has been suggested that hail is rain frozen by being car¬ 
ried in strong ascending currents to higher, freezing altitudes. 
The occurrence of hail near the storm center, where the 
convectional ascent is strongest, supports this claim. 


Fig. 74. —■ Hailstones from Emporia, Kansas 
Photo by United States Weather Survey. 

It is also suggested that the meeting of cold and warm 
masses of air may result in horizontal stratification, warm 
and freezing layers alternating; and that raindrops formed 
near the top of such a stratified cloud are frozen and enlarged 
in passing through it. The addition of snowflakes, formed 
in the colder layers of the cloud, would explain both their 



154 


NEW PHYSIOGRAPHY 



















































































































































MOISTURE OF THE AIR 155 

sponginess and size. Hailstones more than nine inches 
around have been reported. 

Sheet Ice. — Sometimes, in winter, rain falls, but immedi¬ 
ately upon touching the ground or trees it is changed into ice. 
This occurs when the lower air is just above the freezing 
point, while the ground and all solid objects near it, being 
better radiators, have cooled to a temperature below freez¬ 
ing. This is called sheet ice. It is popularly though errone¬ 
ously called sleet. 

During such ice storms ice encases the twigs and boughs 
of trees and shrubs, and sometimes the weight of ice is 
sufficient to break the branches. Such storms are especially 
destructive if strong winds occur before the ice disappears. 
Telephone and telegraph lines are often broken down. 

QUESTIONS 

1. Precipitation increases for a time, then decreases with increase 
of altitude upon the slopes of most mountains. Give reasons. 

2. Why is it that, if we determine the humidity of the air when it 
is raining, we rarely find the air saturated? 

3. The relative humidity decreases, usually, as the day advances, 
until one or two o’clock; explain. Does the absolute humidity de¬ 
crease in the same way? 

4. Why is the relative humidity of the air 10 feet above the 
ground usually higher at night and lower during the day than that 
of the air 100 feet above the ground? 

5. Why is the absolute humidity greater over, and near, the sea 
than inland? 

6. Why does one become so much more quickly chilled in a wet 
garment than in a dry one? 

7. Why, in winter, does frost form on the window panes of an occu¬ 
pied room, but does not, if the chamber is unoccupied? Why will 
thorough ventilation of the occupied room prevent such formation? 

8. Why do fogs form over lakes before they form over the sur¬ 
rounding lands? How are such fogs dissipated as the day advances? 

9. Rain may sometimes be seen falling from clouds, yet not 
reaching the ground. What becomes of it? 

10. Why is there no distinctly wet or dry coast under the equator? 


CHAPTER XI 


LIGHT AND ELECTRICITY OF THE AIR 

Introduction. — We see the sun at rising as a golden 
sphere, at midday a globe of dazzling white; and at setting, 
if the air is dusty, it may disappear below the horizon as a 
ball of fiery red. As we ascend the mountain slope, the noon¬ 
day sun takes on a bluish tinge; and we are told that bal¬ 
loonists, in their highest ascents, see a distinctly blue sun. 

The ever-changing color of the sun, as it mounts toward 
the zenith, or shines through clear or cloudy air, is the re¬ 
sult of the irregular reflection of the light. White light is 
composed of many colors, each having its distinct length of 
ether wave; and because of these different wave lengths, 
white light may be separated into its component colors. 
The shortest waves are blue, and like ripples upon water are 
easily turned aside by obstacles in their path; the longest 
visible rays are red, and these, like the great waves of the 
sea, pass by obstacles which send back shorter waves. 
Hence it follows that an object may appear one color by re¬ 
flected light, and quite a different color by transmitted light. 

A glass of soapy water, viewed from above, looks bluish 
white; but when the sun is seen through the water, its color 
is red or reddish yellow. The short blue waves are turned 
back, whereas the longer reds pass by the minute solid 
particles in the water. 

Color of the Sky. — The dust and cloud particles in the 
air reflect the shorter waves and transmit the longer; and 
the freer the air from dust and cloud, the more completely 
do light waves of all lengths pass through it. To an observer 
looking toward the sun, the irregularly reflected or diffused 

156 


LIGHT AND ELECTRICITY OF THE AIR 157 

light is lost, and only the longer waves reach the eye. If 
the shortest blues alone are scattered, as when the air is 
moderately clear, the combination of the remaining colors 
gives the sun a yellowish tinge; but if the light passes 
through a very dusty air or through a considerable thickness 
of cloud, the sun takes on a distinctly reddish tinge. 

If the eye is turned away from the sun, the diffused light 
is received; and as the blue rays are in excess in diffused 
light, the sky appears blue. The less the admixture of other 
colors with the blue the deeper the blue; and for this reason 
the sky, as seen from a balloon or mountain top above the 
dust-laden stratum of air, appears intensely blue. 

If the air is very dusty, many other colors are diffused 
with the blue, and the sky assumes a whitish glare. The 
sky is bluer at sea and after a rain, because the air is freer 
from dust. It is believed that, if we were to rise above all 
the dust and cloud particles, the sky would have the black¬ 
ness of night; and the stars would shine as brilliantly by 
day as by night. 

Refraction. — When a ray of light passes obliquely from 
a medium of one density to a medium of different density, 
it is bent or refracted at the point of passage. If the passage 
is from a rarer to a denser medium, as from air to water, the 
ray is bent toward the perpendicular, or normal, to the sur¬ 
face separating the two media. 

In Figure 76 the ray RO in the rarer medium takes the direction 
OT in the denser, being bent toward the normal POP'. The angle 
ROP is called the angle of incidence , and the angle TOP' is called 
the angle of refraction. An observer at T sees an object at R in 
the direction OR'. 

The only ray not suffering refraction is one normal to the sur¬ 
face CK. 

If, however, the passage is from a denser to a rarer medium, 
as from water to air, the refracted ray is bent away from the 
normal to the surface separating the two media. 


158 


NEW PHYSIOGRAPHY 


In Figure 77 the ray RO in the denser medium takes the direc¬ 
tion OP in the rarer, being bent away from the normal POP'. The 
angle of incidence is now less than the angle of refraction, and to an 
observer at T the ray seems to come from R'. 

Inasmuch as the lower air is usually denser than that 
above, the ordinary effect of refraction is to increase the 



Fig. 76. — How a Ray of Light Is Bent in Passing 
from Rarer to Denser Medium 


altitude of all heavenly bodies excepting only one situated 
in the observer’s zenith. The nearer the body is to the hori¬ 
zon the more it is displaced vertically by refraction. 

At sunrise and sunset it is sufficient to displace the sun 


p 



Fig. 77.— How a Ray of Light Is Bent in Passing 
from a Denser to a Rarer Medium 


the width of its disk; and as its effect is always to increase 
the altitude of the sun, the entire disk of the sun appears to 
be above the horizon when in reality below it. 

The effect of refraction is to increase the length of the day 
in all latitudes. While this increase amounts to only a few 












LIGHT AND ELECTRICITY OF THE AIR 


159 


minutes at the equator, at the poles it amounts to about 
four days. At the time of the equinoxes the sun is wholly 
above the horizon at both poles. The day at each pole is 
thus lengthened by about four days, the polar night being 
shortened by this amount. Another effect of refraction is 
to give the sun, at rising, an elliptical appearance, flattened 
vertically. 

It is because of refraction away from the normal that an 
observer on a mountain top sees other lower mountains 
higher than they really are; and an observer in the air sees 
the landscape below as a great basin (Figure 78). 

Looming. — Normally the air is densest at the bottom, 


o 



Fig. 78. — Landscape as Seen prom a Mountain Top or Airplane 


becoming rarer with increase of altitude. Sometimes the 
lower air, for a thickness of a few feet, is abnormally cooled 
and denser than the air ten or twenty feet higher. This is 
likely to occur in the early morning in summer, particularly 
over the land, owing to the rapid radiation of the land at 
night and the cooling of the air near it. 

Rays of light coming from an object that rises above the 
cooled stratum of air to an observer within it are bent down¬ 
ward, and the object is seen as occupying a position higher 
than is real. Sometimes objects appear suspended in air, 
in their normal upright position; but more often they are 
merely elongated upward. This is looming. 

In Figure 79 above the surface AB the air to the height of CD 
is abnormally cooled; coldest at the bottom and becoming warmer 











160 


NEW PHYSIOGRAPHY 


and rarer above CD. From the object XZ which rises above CD, 
rays come to the observer at E, within the denser layer of air. These 
rays are bent downward upon entering the denser layer, making the 
part of the object above CD appear to occupy a higher position. If 
the object is distant, only the upper part is seen; but as the object 
is approached, it descends, until finally seen as an elongation of XZ. 


T 



Looming is an early morning or winter phenomenon, and ships 
at sea are often discovered while yet below the observer’s horizon. 

Total Reflection. — When a ray cf light passes obliquely 
from a denser to a rarer medium, the ray is bent away from 
a perpendicular to the surface of the media at the point of 
passage. There is an angle cf obliquity varying with the 



densities, beyond which the ray will not pass from the den¬ 
ser to the rarer medium, but will be totally reflected from 
this surface back into the denser medium. This angle is 
called the critical angle , and this phenomenon is known as 
total reflection. 

In Figure 80 the ray RO in the denser medium takes the direction 
OT in the rarer. If the angle of incidence be increased to R'OP', it is 












LIGHT AND ELECTRICITY OF THE AIR 


161 


called the critical angle, and the refracted ray OT' passes parallel 
to the surface CK. If the angle of incidence be furiher increased as 
R 2 OP', the ray is totally reflected back into the denser medium. This 
phenomenon is known as total reflection, and the bounding surface 
between the two media becomes a mirror. When the surface is 
below the eye of the observer, objects above the surface are seen in 
reflection below their real positions and inverted, as reflections from 
a quiet pool of water. When the reflecting surface is above his eye, 
the images are suspended in air and inverted. 

Hot Weather Mirage. — The interesting phenomenon of 
mirage depends upon the principle of total reflection in the 
air. Often in deserts or upon arid plains during a hot sum¬ 
mer day the air resting upon the land becomes greatly 



heated, and rarer than the air above. If the day is calm, 
considerable difference in density may be developed be¬ 
fore convectional [motionsets up. When this condition of 
things exists, travelers often see distant objects reflected 
as from a water surface. In reality the reflection is from 
the upper surface of the rare stratum of air. 

In Figure 81, CD is the upper surface of a thin, rare layer of air 
overlying AB, and distinctly rarer than the air above. A distant 
object, XZ, rises above CD. The angle at which the rays from the 
object strike the surface CD is greater than the critical angle, and the 
rays are totally reflected. The object is then seen in the position 
XZ', as reflected from a water surface. The object and its image are 
both seen. 

This species of mirage is peculiarly a land phenomenon. It is 
seen in hot weather, and oftenest in low latitudes. 







162 


NEW PHYSIOGRAPHY 


It may often be observed in the morning above an asphalt 
street. The asphalt warms up much faster than the air 
above it, and the thin layer of air in contact with the asphalt 
is heated and reflects cars at such a distance that rays 
from them strike the reflecting surface at an angle greater 
than the critical angle. The street appears to be wet. 

This phenomenon, most commonly observed in deserts, 
often misleads caravans unused to it to think they are ap- 


X' 



AP'B is the curved surface of the earth. Object below the horizon. 

proaching a body of water, as such reflections from water sur¬ 
faces are known to all. Hills and mountains in the distance 
are seen together with their reflections, only to have the 
mirage fade upon nearer approach, when the angle of in¬ 
cidence becomes less than the critical angle. 

Cold Weather Mirage. — Another species of mirage is seen 
in winter or early morning, when the lower air is for a con¬ 
siderable depth colder and denser than the air above. The 
upper surface of the cold stratum of air then serves as a 
mirror from which objects below are reflected. 

This is illustrated in Figure 82. Rays from a distant object, such 
as a building or ship, strike the surface CD at such an angle that 






LIGHT AND ELECTRICITY OF THE AIR 163 

they are totally reflected, and the image of the object is seen above 
its real position and inverted. The object reflected may be, and 
usually is, invisible, because below the horizon. 

This is a common phenomenon in the Atlantic waters off 
the coast of Greenland, where the cold air from the glacial 
ice sheet sinks down and spreads far out over the sea. 
“Phantom'’ ships are seen, masts down, sailing through the 
air, and no ship visible upon the water. The statehouse at 
Baton Rouge, Louisiana, was thus seen suspended and in¬ 
verted, from Clinton, a village 30 miles north of Baton 
Rouge. 

Dispersion of Light. — White light is known to be a mix¬ 
ture of several colors of light, all traveling together at the 
rate of 186,000 miles a second, but of different wave lengths. 
If we consider but three of these colors, red, yellow, and 
blue, the red has the longest wave, the yellow next, and the 
blue the shortest. 

When a ray of white light enters a drop of water or crystal 
of ice, it is usually refracted, the several colors being un¬ 
equally bent. On this account the colors are separated or 
dispersed. The shortest wave lengths, the blue, are bent 
most, and the red is bent the least. In passing out of the 
refracting medium, the colors are still further bent and more 
separated. On this account but one color can be received 
by the eye from any water drop or ice crystal. 

Halos. — Sometimes, in front of a cyclone, when the 
cirrus clouds that outrun and foretell the coming storm 
stretch across the sky, light or colored rings encircle the 
sun or moon. These are called halos , and result from disper¬ 
sion of the colors as the light passes through the ice crystals 
that make up the cirrus cloud. The red, since it is least 
bent, will occupy the inside of the ring, and the blue the out¬ 
side. As the cloud sheet thickens with the nearer approach 
of the storm, the ring becomes smaller. There is thus 
scientific basis for the very general belief that the greater 


164 


NEW PHYSIOGRAPHY 


the number of stars to be seen inside the ring, the greater 
the number of days before the arrival of the storm center. 
In reality, of course, it is the size of the ring, not the ac¬ 
tual number of stars, for the position of the moon among 
the stars would determine how many stars a given-sized 
ring would contain. 

The colored rings seen about a street light when one looks 
through a frost-covered window pane are similar to the halos 
seen about the sun and moon. The tiny water particles 
that constitute a fog also cause a similar dispersion of the 
colors of light, but the nearness of these particles to the ob¬ 
server causes the rings to lie very near the light, and these 
are generally called coronas rather than halos. They are 
both due to like cause. 

The Rainbow. — If an observer stands with his back to 
the sun while rain is falling in front of him, there often ap¬ 
pears the arc of a circle, made up of parallel bands of colored 
light. This is the rainbow. It results from the dispersion 
and total reflection of light by raindrops so situated that the 
reflected ray comes to the eye of the observer. But one 
color comes from any drop, and the drops are so situated 
that lines drawn from them to the eye make an angle of 
about 41° with a line passing through the eye and the center 
of the rainbow. This is the primary bow; and unlike the 
halo, the red is on the outside of the arc and the blue on the 
inside. 

The greater the altitude of the sun, the shorter the arc. 
Thus the rainbow is rarely seen at midday. It appears as a 
half-circle at sunrise and sunset. It may be seen as a com¬ 
plete circle from a mountain top or by observers in the air. 
Similar bows may be seen in the spray of waterfalls and in 
playing fountains. 

Though commonly a daytime phenomenon, rainbows are 
sometimes produced by moonlight. If situated with a sheet 
of calm water at his back, one may see two rainbows pro¬ 
duced by direct rays from the sun and by rays reflected 


LIGHT AND ELECTRICITY OF THE AIR 


165 


from the water. The bow produced by the reflected rays 
lies above the other, and is longer, being sometimes a full 
circle. 

A less distinct secondary bow, outside the primary and 
with the order of the colors reversed, is sometimes seen. It 
is produced by raindrops so situated that the light is twice 



Fig. 83. — Photograph of Lightning Flash 


reflected within the drop before passing out to the eye of the 
observer. The diameter of the secondary bow is about 100°. 
It is never as distinct as the primary bow. 

Under most favorable conditions a third bow outside of 
the secondary is faintly seen. The order of the colors is the 
Same as in the primary. It is caused by light three times 
reflected within the drop before passing out to the eye. 

Lightning. — Every year records a considerable loss of 






166 


NEW PHYSIOGRAPHY 


life and property in the United States from lightning. Men 
and animals are killed, trees and houses shattered and often 
set on fire, and hay and grain stacks burned. Lightning 
storms, commonly called thunderstorms, are usually asso¬ 
ciated with an overheated condition of the air, locally, and 
are most common in the afternoon of hot summer days. 
They occur not infrequently at night, and may even occur 
in winter; but their cause seems always to be found in a 
rapid convectional overturning of the lower air. 

The air is always electrified, but it is only when clouds are 
forming rapidly, as in the ascending currents near some 
storm center, that discharge takes place. This discharge, 
known as the lightning flash, may be between clouds, or it 
may be from cloud to earth. When the discharge is down¬ 
ward, those objects which rise highest, as buildings and trees, 
are most in danger of being struck by lightning, since they 
are better conductors than air. 

In those sections of the country where wire fences are ex¬ 
tensively used, great numbers of stock are annually killed 
by lightning, because they sometimes collect near and touch 
these fences during thunderstorms. The wire serves as a hor¬ 
izontal conductor, often for considerable distances. The elec¬ 
tricity is finally led to the ground by the fence posts, which 
are thereby shattered. Telegraph and telephone poles are 
shattered in a similar fashion. The belief that “ lightning does 
not strike twice in the same place” is a dangerous error. 
The fact that lightning strikes in any given place argues the 
existence there of favorable conditions. 

Protection from Lightning. — The fear of lightning and 
the desire for immunity from it have led to the adoption of 
many protective measures. Perhaps the most common arti¬ 
ficial protection is the lightning rod. 

As all solids are better conductors of electricity than the 
air, buildings and trees are more often struck by light¬ 
ning than the open surface of the ground near them. The 
greater the number of buildings or trees among which the dis- 


LIGHT AND ELECTRICITY OF THE AIR 


167 


charge may be divided, the less the individual liability. On 
this account a house in the city has greater immunity from 
lightning than the isolated farmhouse; and any tree in the 
forest is safer than the “lone tree” upon the prairie. 

Where country houses are surrounded by shade trees these 
offer perhaps the best protection from lightning. Each indi¬ 
vidual tree becomes a means for silently discharging the 



Fig. 84. — Tree Destroyed by Lightning 


passing clouds of their electricity, thus preventing heavy and 
destructive discharges. 

The lightning rod as a protection against lightning has 
been in use almost ever since the discovery of the nature of 
lightning. Its use is based on the theory that metal, being 
a better conductor of electricity than the building, by afford¬ 
ing an easier route, will prevent the discharge from passing 
to the building itself. It is also thought that the numerous 
points which rise above the building may quietly drain 
away the electric charge from the clouds, and thus prevent 
a destructive discharge. 






168 


NEW PHYSIOGRAPHY 


The lightning rod usually consists of a metal ribbon or 
flattened tube, commonly of copper or galvanized iron, laid 
over the roof of the building, with frequent branches rising 
from six to ten feet in air. These branches end in one or 
more sharp points; and the rod should extend sufficiently 
deep into the ground to reach moist earth. If it ends in 
a cistern or well, so much the better. The greater the num¬ 
ber of branches rising above the building the better, as the 
discharge is thereby more divided. 

Perhaps the greatest security from lightning is obtained 
by encasing the structure in a network of wire. 

Kinds of Lightning. — Zigzag lightning is a very common 
form. The course of the flash is probably a sinuous one 
and appears angular only when seen along the direction 
of its path. Ramified or branching lightning may begin 
and end in a multitude of branches, uniting in a trunk 
flash between. This kind of flash takes place between 
clouds. Heat lightning and sheet lightning, probably the 
same, are believed to be the illumination of cloud masses 
so far away that the accompanying thunder is not heard. 
St. Elmo’s Fire is the name given to the discharge of 
atmospheric electricity as a brush of bluish flame, often 
observed at the ends of masts and spars of ships, at tree 
tops, or house tops, or any pointed object. A crackling 
sound accompanies it similar to that of the artificially pro¬ 
duced electric spark. 

Thunder. — The production of thunder may be likened 
to the production of the noise accompanying the explosion 
of gunpowder. The lightning flash heats the air along its 
path and pushes it away. The collision of the returning air 
particles causes a high-pitched crackling in the case of a few 
sparks; but when the sparks are longer, the collision of 
the returning air particles causes a low-pitched roar. The 
quick succession of crashes following along the path of 
the flash unite to produce the roar; and the sound, when 
echoed and reechoed from cloud masses, gives the rolling 


LIGHT AND ELECTRICITY OF THE AIR 


169 


so often observed to follow brilliant flashes of lightning 
during thunderstorms. 

Relation of Lightning to Rain. — The condensation of the 
moisture in the air forms cloud particles, and the merging of 
these particles, by reason of their mutual attraction, forms 
raindrops. With increase in size of the raindrops the elec¬ 
tric charge of the drop is increased, and lightning discharge 
is made possible. 

It would seem that lightning for the most part precedes 
the rain. When the raindrops begin to fall, each carries 
down with it a minute charge, and in this way the cloud 
mass is discharged. Further production of lightning is then 
impossible. The heaviest fall of rain in a thundershower 
often follows closely the most brilliant lightning and heaviest 
thunder. So while lightning mainly precedes the rain, it 
seems probable that they are mutually cause and effect. 

The Aurora. — This is the beautiful electrical display, 
common in high latitudes, though often seen in northern 
United States. In the northern hemisphere it is called 
aurora borealis, and in the southern, aurora australis. It is 
believed to be due to the discharge of electricity into the 
rare upper air and seems to bear some relation to sun spots. 
The aurora lessens the gloom of the long polar night. It is 
of such frequent occurrence and so brilliant in arctic lati¬ 
tudes that the nights are rarely dark. 

As seen in the United States the aurora, also called north¬ 
ern lights, usually consists of a more or less distinct arch 
of light, extending east and west, which is crossed at right 
angles by streamers of colored light radiating from a point 
in the northern horizon. The arch is highest above the mag¬ 
netic meridian. The streamers of red, yellow, and green 
light change their position and length so rapidly that they 
are called “the merry dancers.” They radiate from the 
north magnetic pole. 

Brilliant aurora displays are often accompanied by severe 
electrical and magnetic disturbances throughout the coun- 


170 


NEW PHYSIOGRAPHY 


try. The telegraph and telephone services are often in¬ 
terrupted for hours, and the magnetic compass sometimes 
becomes so variable as to be useless. 

QUESTIONS 

1. Why do we not ordinarily observe the phenomenon of “loom¬ 
ing ” at midday? , 

2. Would the phenomenon of “looming be dispelled by ascend¬ 
ing into the air? . 

3. In the mirage which results from the lower air bemg cooler 
than that above, need the object, the image of which is seen, be 

visible? _ . , , 

4. It is a common notion that the greater the number of stars 
visible within the halo rings about the moon, the greater the num¬ 
ber of days before the storm, which these rings presage, will arrive. 
What scientific grounds exist for this belief? 

5. How did Franklin discover the identity of lightning with the 
artificial electric spark? 

6. Why should we avoid the shelter of tall trees m a thunder¬ 
storm? Why do we disconnect our telephones during thunder¬ 
storms? 


CHAPTER XII 


WEATHER AND CLIMATE 

Weather and Climate Defined. — Weather is the condition 
of the air at a given time and place with reference to tem¬ 
perature, pressure, moisture, state of the sky, and winds. 
These conditions, called weather elements, are constantly 
changing; and as a consequence, for most places in tem¬ 
perate latitudes, the weather is proverbially fickle. 

After sunrise, as the day advances, the temperature nor¬ 
mally rises, reaching its maximum between one and three 
o’clock p.m., after which it falls till near sunrise the follow¬ 
ing day. With these changes in temperature come changes 
in the relative humidity, and usually changes in pressure, 
wind direction, and strength. 

Climate is the average condition of the air with reference 
to temperature, pressure, moisture, state of the sky, and 
winds; or it is average weather. When observing the wea¬ 
ther, we take account of current temperatures; but in con¬ 
sidering climate, we find that maximum and minimum 
temperatures are more important. 

We use the term weather in referring to atmospheric con¬ 
ditions at any given instant; also for such short periods 
as a day, a week, or a month. We even speak of “summer” 
or “winter” weather; but when we apply the term to 
these longer periods we refer rather to the average conditions 
during these periods. 

Weather Changes. — Although the variability of the 
weather is proverbial, yet there are important controls, 
which, by reason of their orderly sequence, give a certain 
degree of system to the succession of weather changes. The 
most important of these are: 

171 


172 


NEW PHYSIOGRAPHY 


1. The alternation of day and night because of rotation. 

2. The annual succession of winter and summer because of revo¬ 
lution. 

3. The more or less systematic passage of lows and highs. 

The first two are fairly regular in period and value at any 
place, though widely differing for different places; whereas 
the third varies in both period and intensity. 

The daily and annual changes of the weather are more 
pronounced near sea level than at higher altitudes, in the 
interior of the continent than near the coast, and in high 
than in low latitudes. As a rule the temperature rises and 
the absolute humidity increases during the day and in sum¬ 
mer, both being lower at night and in winter. 

Convectional cyclones are more frequent over the land 
than over water, more vigorous in summer than in winter, 
and more vigorous also in the daytime than at night; whereas 
nonconvectional cyclones are most frequent and intense 
in winter. Both classes of Cyclone are probably more highly 
developed and likewise of longer duration over the sea than 
over the land. This is due to the greater humidity of the 
air over the sea. The more abundant condensation of water 
vapor in the ascending currents releases a greater amount 
of latent heat, and this in turn promotes further ascent of 
the air, thus intensifying the cyclonic conditions. 

In the United States during .March and April, when the 
land is warming up most rapidly, it is a common occurrence 
to have days of blustery winds succeeded by nights of calm. 
This is due to the rapid warming and convectional over¬ 
turning of the lower air during the day. At night, when the 
lower air becomes colder than the air above, convectional 
overturning ceases and the winds die down. 

Weather in the Tropics. — Night has been called the 
“winter of the tropics.” This is because the variation in 
weather conditions from day to night is greater than from 
winter to summer. 


WEATHER AND CLIMATE 


173 


In the doldrum belt the days are uniformly warm, owing 
to the nearly vertical rays of the sun; and the rapid convec- 
tional ascent of the air in the morning is usually followed 
later in the afternoon by torrential downpours of rain, 
followed in turn by cloudless nights. The nearly equal day 
and night, combined with the low percentage of cloudiness, 
accounts for the great daily range of temperature. Cyclonic 
interruptions are of secondary importance. 

In the trade-wind belts, over the sea, there is a constancy 
of weather conditions not found elsewhere. The extreme 
range of temperature scarcely exceeds 10°; and the wind 
blows continuously from the same direction and with about 
the same strength day and night. On land the range of tem¬ 
perature increases, and over both land and sea there is little 
rainfall, except where the winds are compelled to rise over 
the ascending land. The slight rainfall is due to the fact 
that the trades grow warmer as they advance. 

Though there is some variability of wind direction and 
some precipitation in the trade-wind belts owing to the 
cyclones which develop there, the constancy of the wind and 
the deficiency in rainfall are the marked characteristics of 
this belt. Continents and islands and, to a more marked 
extent, mountain ranges that lie across the path of the 
trades are usually well supplied with rain. As illustrations 
of this, note the abundant rainfall of eastern Australia, east¬ 
ern Africa, and Brazil; the northern slopes of the West 
Indies and the Hawaiian islands; and the eastern. slopes 
of the Andes and the mountains of Mexico. In all these 
cases the winds are forced to rise and are cooled. 

Regions on the borders of the trades have monsoon 
changes of weather. If these regions are next the doldrums, 
there is the alternation of the light winds and abundant rains 
of the doldrums and the constant winds and light rains 
of the trades. If they lie next the high-pressure calms of 
the horse latitudes, then the characteristic conditions of the 
trades alternate with those of the horse latitudes. 


174 


NEW PHYSIOGRAPHY 


Weather Outside the Tropics. — In the zone of prevail¬ 
ing westerlies weather changes are irregular and mainly of 
cyclonic control with marked differences in the two hemi¬ 
spheres. In the southern hemisphere, where there is little 
land to interrupt them, the westerlies attain a constancy 
approaching that of the trades and so high a velocity 
that they are called the “ Roaring Forties.” In winter the 
cyclones are more frequent and succeed each other with 
almost periodic regularity. 

In the northern hemisphere, where the land is massed, 
there is a strong contrast between the weather of the land and 
the water areas of the prevailing westerlies, the land 
areas having much greater extremes of weather conditions, 
both daily and seasonal. The greatest seasonal ranges of 
temperature are found in the interiors of the northern con¬ 
tinents. In northern Siberia there is a range of about 200°F. 

As a result of the massing of the continents in the northern 
hemisphere, the North Atlantic and North Pacific oceans 
are low-pressure centers during the northern winter and high- 
pressure centers during the northern summer. Over north¬ 
ern America and Eurasia the pressure is low in summer and 
high in winter. It is from these seasonally permanent pres¬ 
sure centers that the cyclones of lower latitudes are projected. 

In the frigid regions, although temperature changes are 
determined chiefly by the appearance and disappearance of 
the sun, the other weather elements are controlled mainly 
by the passage of cyclones. The precipitation, though less 
abundant, is mostly in the form of snow which accumulates 
upon the land. If more snow falls than disappears by melt¬ 
ing and evaporation, its accumulation results in the forma¬ 
tion of an ice sheet like that which covers Greenland and the 
Antarctic continent. 

Weather Prediction. — After a thorough understanding of 
the relative values of the factors determining weather in 
any region, it is possible to predict, with a high degree of 
accuracy, the changes of weather likely to occur. The degree 


WEATHER AND CLIMATE 


175 


of accuracy attainable varies with the season and with geo¬ 
graphic position. Under the doldrums and trade winds, 
where the diurnal change is dominant, weather prediction 
may be made with an assurance almost amounting to a 
certainty that it will be fulfilled. Indeed, the weather 
changes there are so regular and certain that the weather 
is not a topic of conversation. 

In regions where the control of the weather is mainly 
cyclonic, it is not possible to predict with nearly so high a 
degree of accuracy. Yet even here the relative values of 
the factors are so well known and the systematic movement 
of cyclonic disturbances so well understood that predictions 
may be made with the reasonable expectation that a large 
percentage of them will be fulfilled. These predictions for 
any station must take account of: (1) the systematic 
movement of cyclonic disturbances, their strength of develop¬ 
ment and place of origin, and direction and rate of move¬ 
ment; (2) the season; and (3) local topography. 

Weather in the Cyclone. — To understand the weather 
conditions which prevail about lows and highs, it is neces¬ 
sary to remember the directions of the winds about these 
disturbances and the effect upon the humidity of the air 
resulting from a change of temperature. 

In the United States, cyclones, as we have seen, move east¬ 
ward, and the winds blow in toward the cyclonic center in 
counterclockwise spirals. At any station the wind will not, 
as a rule, be blowing directly toward the center, but a little to 
the right of it. Therefore, in front of the cyclone the winds 
are blowing from a warmer to a cooler latitude, and their 
relative humidity is increased. As they approach the center 
of the low, the air rises, and its humidity is further ip- 
creased by cooling from expansion. This may be sufficient 
to bring the air to saturation. As a result of these conditions 
a rising temperature, with cloudiness or precipitation, generally 
characterizes the front of the low and may be predicted as a 
well-developed cyclone approaches. 


176 


NEW PHYSIOGRAPHY 


In the rear of the cyclone the winds are moving from colder 
into warmer regions, and as a result the relative humidity 
of the winds is lowered. As they near the center of the low and 
begin to rise, their temperature falls as a result of expansion; 
but the cooling must first counteract the warming due to 
their moving into warmer latitudes before their relative 
humidity reaches that possessed by the winds when they 
were inaugurated. 

As a result of this difference in conditions in front of 
and behind the cyclone, the increase in humidity, due to as¬ 
cent, may bring the air in front of the cyclone's center to 
the saturation point, and yet not saturate the less humid 
air in its rear. 

Consequently falling temperatures and clearing skies may be 
expected after the center of a well-developed cyclone passes. 
In the tropics the rainfall in the rear of the cyclone exceeds 
that in front of it. Why? 

The direction of the shifting of the wind depends, as we 
have seen, upon the position of the path of the cyclone’s 
center, whether north or south of the station. Ordinarily 
the strength of the wind increases as the cyclone approaches 
and decreases as the cyclone recedes. 

In winter the strong indraft of cold air in the rear of a cy¬ 
clone, if accompanied by snow, is known in the United States 
as a blizzard. 

Weather in the Anticyclone. — Since the movements of 
the air about a high are the reverse of its movements about 
a low, it follows that the conditions as regards temperature 
and humidity which prevail about a high are likewise the 
reverse of those which prevail about a low. In front of a 
high the winds are northerly, and behind a high the winds 
are from some southerly quarter, while at the center of the 
high the air is sinking. Consequently fair and cooler weather 
is predicted, usually, as the high approaches, and rising tem¬ 
peratures with possible cloudiness or even precipitation as 
the high recedes. 


WEATHER AND CLIMATE 


177 



Fig. 85. — An Ideal Low 

Note that (1) the heavy arrow indicates the movement of the low toward the north¬ 
east and that (2) the isotherms cross the low in a northeast-southwest direction instead 
of following their usual east-west path. Note also (3) the cirrus overflow in advance 
of the rain cloud and (4) clearing skies in the northwest quadrant of the low. 



Note that (1) the heavy arrow indicates the movement of the high toward 
the northeast and that (2) the isotherms follow a northwest-southeast 
course through the high-pressure area instead of following their usual east- 
west course. (3) Clearing skies precede and cloudiness follows the passage 
of the center of the high. The center of the high is fair or clear and cool. 















178 


NEW PHYSIOGRAPHY 



Since the winds start from the center of the high, unlike 
the low, the winds weaken as the high approaches and 
strengthen as it recedes. As with the low, the direction of the 
shifting of the wind is determined by the position of the 
station with reference to the path of the center of the high. 

In winter, if a high follows closely in the wake of a well- 
developed low, the fall in temperature may be abnormal. 


Fig. 87 . — Vertical Section of a Thunder Squall 

The squall is moving from left to right directly toward the wind. As it approaches 
and before the rain, there is a lull in the wind, and then a sudden change m direction 
to that from which the squall approaches. This outrush of air shown by arrows and 
letters is probably produced by the falling rain that quickly follows. Above the 
cumulus cloud may be seen the cirrus overflow. (After Koeppen.) Inflowing winds, 
a, b, c, in front of squall, OP, in trough of the air current covered by falling rain. 

If it is as much as 20° F. in 24 hours, reaching a temperature 
of freezing or lower, it is called a cold wave. In southern 
United States the term is applied to changes somewhat less 
than 20°, reaching a somewhat higher minimum. 

Thunderstorms and Tornadoes. — In summer, after a 
day or so of excessive heat, the rapid convectional ascent of 
the air about a low may set up, locally, a more limited 
though more intense cyclonic whirl. The rapid condensa- 




WEATHER AND CLIMATE 


179 


tion of vapor in the rising and cooling air may give rise to, 
or be accompanied by, brilliant displays of lightning and 
heavy thunder. Such storms are known as thunderstorms. 
Torrential downpour of rain may follow quickly after the 
most brilliant discharges of lightning, but it is a notable fact 
that the lightning flashes become rapidly less frequent after 
the rain begins to fall. 



Fig 88. — Photograph of a Tornado near Oklahoma City, Oklahoma 
Photo by Wide World. 


Thunderstorms are usually summer and daytime phe¬ 
nomena, though they sometimes occur in winter and at night. 
They are much more common in front of lows than behind 
them. In the United States they occur most frequently in 
the southeastern quadrant of the low-pressure area. 

If the local whirl thus developed is destructive in vio¬ 
lence, it is called a tornado. The destructive path of a tor¬ 
nado is rarely a mile in width and usually but a few miles 
in length; more commonly it is but a few hundred yards in 
width. Within that narrow path the violence of the wind 





180 NEW PHYSIOGRAPHY 

is such that few structures above ground are strong enough 
to withstand it. In those states in the Middle West where 
tornadoes are most frequent, underground structures called 
“cyclone cellars” are built. These seem to offer the greatest 
security from danger. 


Fig. 89. — What the Same Tornado Did in Texas 
Photo by Underwood & Underwood. 

Tornadoes progress normally in a northeastern direc¬ 
tion, at a rate of 20 or 30 miles an hour, whereas the 
spiraling winds about the tornado center may attain a ve¬ 
locity of more than 100 miles an hour. Tornadoes are 
most frequent in the afternoon of hot summer days and 
seem to need for their development a fairly level land surface; 
hence we do not have them in mountain regions, nor do 
they occur upon the Pacific coast. The broad, level Missis¬ 
sippi Valley seems best suited of all places in the United 
States for their development. 

Weather Service. — For the purpose of a more thorough 
study of the weather and more accurate prediction of wea- 








WEATHER AND CLIMATE 


181 


ther changes, the United States Government has established 
a weather service extending to all settled parts of the coun¬ 
try. This service, which is the work of the Weather Bureau, 
a division of the Department of Agriculture, has its central 
office in Washington, D. C. Its corps of observers, paid 
and voluntary, to the number of more than three thousand, 
are distributed throughout the country. Regular observa¬ 
tions of the weather are made at more than two hundred 
stations, as nearly as possible at the same instant, 8 a.m. 
and p.m., 75th meridian time, and are reported by tele¬ 
graph to the central office at Washington and to each other. 
The most important observations are: pressure; tempera¬ 
tures, current, maximum, and minimum; direction and 
strength of the wind; amount and kind of precipitation during 
the past 24 hours, and percentage of cloudiness. 

Weather Maps. — When these data are collected and 
plotted on a map of the United States, the result is a weather 
map. The daily weather map is published at the central 
office in Washington and also at one or more substations in 
every state. It not only sets forth the weather conditions 
existing at the time of observation, but also serves as a basis 
of prediction of the weather for the 24 or 36 hours following. 
Each local map supplements the general prediction for the 
entire country with a forecast for the particular locality. 

To be of value for purposes of forecasting, weather maps 
must be distributed the day issued, since weather conditions 
are constantly changing. 

Since our weather is mainly of cyclonic control and since 
the cyclonic disturbances move eastward across the country, 
the weather map as a basis of weather prediction is of more 
value to the eastern than to the western part of the country. 
On the Pacific coast it is of little value, since there are few 
stations farther west to report coming changes. With fur¬ 
ther extension of wireless telegraphic service and the radio, 
the value of the weather service to our western coast will 
be correspondingly enhanced. 


182 


NEW PHYSIOGRAPHY 


Value of Weather Predictions. — Every observer is fa¬ 
miliar with the daily and seasonal changes of temperature ) 
also with the fact that there are other almost equally impor¬ 
tant changes that are irregular in their occurrence. More 
and more people are learning to appreciate the relation of 
these unperiodic changes of the weather to the eastward 
march of cyclonic disturbances and to appreciate the great 



Isotherms, dotted lines drawn for every 10°; and isobars, unbroken lines drawn for every 
tenth of an inch. Line of arrows indicates the ordinary path across the United States 
of this type of low. Such lows usually advance at the rate of about 30 miles an hour. 


value of our weather predictions. Each year brings a wider 
use of these predictions and a more general rejection of the 
predictions of charlatans who make year-long forecasts. 

Among the first to realize the benefits of our weather fore¬ 
casts were the shipping interests of our southern and east¬ 
ern coasts and of the Great Lakes. Not infrequently 
censuses have shown that marine property to the amount of 
more than $25,000,000 has been held in port because of 





































































WEATHER AND CLIMATE 183 

storm warnings issued. Few masters of vessels now leave 
port without knowing the latest forecast of the weather. 

Shippers of perishable goods are also interested in weather 
predictions. Estimates from shippers place the value of 
property saved by the warning of the cold wave of January 1, 
1898, at nearly $5,000,000. Farmers, planters, truck grow¬ 
ers, and fruit growers are interested in being forewarned of 
changes in the weather, especially when these changes mean 
destructive winds, floods, or frosts. A wider use and appre¬ 
ciation in all fields of the great value of weather predictions 
is developing with the wide extension of the use of the tele¬ 
phone and of rural mail delivery. 

It is our confident belief that, with a more extended field 
of observation and a better knowledge of upper air condi¬ 
tions, the present practical limit for safe predictions of 36 
hours may be considerably increased. By means of kites 
and balloons the upper air is being explored. 

Weather Signs and Proverbs. — There are two distinct 
classes of weather signs. The first are based on century- 
long observations by those whose occupations have led them 
to observe weather changes closely; the second class in¬ 
cludes a mass of superstitions that have been strangely 
preserved and transmitted. The signs of the first class have 
usually found expression in trite sayings that have come to 
be known as weather proverbs. As an aid to memory these 
proverbs are commonly expressed in rhyme. 

Weather proverbs are usually of only local application, 
though many are worldwide. When local, in order to appre¬ 
ciate them, one must be acquainted with the local conditions. 

“Rainbow in the morning, sailors’ warning; rainbow at night, 
sailors’ delight” is a proverb that is true only in those regions where 
cyclonic storms move eastward. If the rainbow is seen in the morn¬ 
ing, the storm center is apt to be westward, and its further progress 
will bring it nearer. 

“Mackerel scales and mares’ tails make lofty ships carry low 
sails” is applicable the world over. The long, wispy clouds called 


184 


NEW PHYSIOGRAPHY 


“mares’ tails,” and the sky flecked with cirro-cumulus clouds, and 
known as a “mackerel sky,” are the result of the high-level overflow 
of air in front of a cyclone. Consequently they presage a coming 
storm. “Mist rising o’er the hill brings more water to the mill” the 
world over. 

Climatic Controls. — Since climate is but average wea¬ 
ther, those conditions which control weather likewise con¬ 
trol climate. The most obvious, and perhaps the most 
important, climatic controls are: latitude, height above sea 
level, distance from the sea, 'position with reference to moun¬ 
tain ranges and with reference to prevailing cyclonic paths. 

Although climate is defined as the average condition of the 
air with reference to the various climatic elements, it does 
not follow that when these averages are the same the climates are 
alike or even similar. 

New York City and San Francisco have about the same 
average temperature for the year, but New York has hot 
summers and cold winters, whereas San Francisco has equable 
temperatures throughout the year. The central Missis¬ 
sippi Valley has about the same annual rainfall as the 
coast of California; yet in the interior the rains are distrib¬ 
uted through the year, but on the coast they are confined 
to the winter months. 

Of vastly more importance than averages are the extremes 
of climatic conditions and the distribution of these condi¬ 
tions through the year. 

Climatic Zones. — Temperature being the most impor¬ 
tant climatic element and depending, as it does, mainly 
upon latitude, the earth may be divided into east-west 
zones, each of which furnishes a distinct type of climate. 
Within any zone there may be considerable variation from 
the type, yet there is sufficient similarity to justify the divi¬ 
sion into zones. 

The customary division whereby the zones are bounded by 
parallels gives us light zones rather than climatic zones; 
therefore, the tropics and polar circles are not boundaries 


Fig. 91. — Climatic Zones 

Note the widening of the temperate zone in the northern hemisphere over the continents and its narrowing over the oceans. 


WEATHER AND CLIMATE 


185 





















186 


NEW PHYSIOGRAPHY 


for torrid, temperate, and frigid climates. A more reasonable 
boundary is the isotherm. It has been suggested that the 
average annual isotherm of 68° F. be taken as the poleward 
boundary of the torrid zone, and the summer isotherm of 
50° F. as the poleward boundary of the temperate zones. 

The temperature of 68° F. is about the temperature we 
desire for our houses in winter, and the temperature neces¬ 
sary for so-called tropical plants; a temperature of 50° F. is 
necessary for trees and for the maturing of the hardier cereals. 
Warm temperatures during the growing season are more im¬ 
portant than low temperatures during the dormant season. 

The temperate zone is the widest zone, and wider in the 
northern than in the southern hemisphere. This is due to 
the excess of land north of the equator, land being a better 
absorber of insolation than the sea. The frigid zones, or 
more accurately the polar cold caps, have a temperature, 
even in the hottest month, below 50° F. 

The Torrid Zone. — As its name suggests, the most dis¬ 
tinctive characteristic of the torrid zone is its high tempera¬ 
tures. In every part, except where mountains rise into cold 
altitudes, the daily maximum is from 75° to 100° F. and often 
higher. But the other climatic elements vary so widely in 
this zone as to justify its division into three parts: 

1. The belt of doldrums or equatorial calms is a belt of 
high temperature, low pressure, light and variable winds, and 
abundant rainfall. The almost vertical rays of the sun heat 
up the lower air in the early forenoon and cause rapid con- 
vectional currents. These rise by noon to such altitudes 
that their cooling by expansion produces condensation of 
their vapor, the formation of clouds, and, in the early after¬ 
noon, rain. The rains are thus of almost daily occurrence 
and abundance throughout the year. More abundant rain¬ 
fall and a higher percentage of cloudiness are to be found 
over the sea than over the land, because of the higher humid¬ 
ity over the sea. 

The days vary little in length, and twilight and dawn are 


WEATHER AND CLIMATE 187 

of short duration, owing to the nearly perpendicular position 
of the sun path to the horizon. 

In this belt occur the dense forests of South America, 
Africa, and the East Indies. 

2. The trades, like the doldrums, have a prevailingly high 
temperature, but unlike the doldrums they have little rain¬ 
fall. Their most marked characteristic is the constancy of 
their winds. These blow day and night, winter and summer, 
with a constancy of both direction and strength equaled in 
no other zone. In the northern hemisphere they are north¬ 
east winds, and in the southern hemisphere southeast. They 
run to the doldrum belt, where the air rises. 

On land the climate of the trades depends upon the direc¬ 
tion of slope, the eastward and westward slopes having un¬ 
like climates. The winds being forced up the eastward 
slopes may yield abundant rainfall, as upon the eastern 
coasts of Central America, Brazil, Africa, and Australia, 
whereas the descending winds upon the westward slopes 
yield little or no rainfall, as shown by the dry western coasts 
of these countries. 

The eastward and northeastward slopes of the mountain¬ 
ous islands of the West Indies and the Hawaiian group have 
abundant rainfall and are heavily forested, whereas the 
southwestern slopes have deficient rainfall and in some cases 
are almost desert. 

Any low-lying land area, island, or continent under the 
trade winds is likely to be desert because of its slight rain¬ 
fall. The great deserts of Africa and Australia are trade- 
wind deserts. The winds moving toward the equator are 
warmed, and their capacity for water vapor is increased; con¬ 
sequently they not only yield little rainfall, but they also 
absorb the moisture of the regions over which they blow. 

3. With the shifting of the wind belts, regions near the 
border line of the doldrums and trades lie alternately under 
these belts. Such regions have seasonal changes of climate 
and are known as monsoon belts. If near the equator, they 


188 


NEW PHYSIOGRAPHY 


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Note the well-developed low-pressure area over the northern ocean and the equally 
well-developed high-pressure area over the southern ocean. Account for this. Long 
arrows indicate steady winds, and heavy or double arrows indicate strong winds. 
















































































































































WEATHER AND CLIMATE 


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Observe that the low in the northern ocean has disappeared and a well-developed 
high appears in the belt of horse latitudes. Find a reason for this. The high contin¬ 
ues, though weakened, over the southern ocean. 






















































































































































































190 


NEW PHYSIOGRAPHY 


have two distinct wet seasons when under the doldrums, and 
two dry seasons when under the trades. 

While these transitional or monsoon belts extend around 
the earth, they are most pronounced where, as in India, the 
monsoon winds are reenforced by the continental winds, owing 
to the great continental mass lying poleward from them. 
When the southwest monsoon blows over India, the “hooked” 
southeast trades are strengthened by the continental in¬ 
draft toward central Asia; and when the northeast monsoon 
blows, the normal northeast trades are strengthened by the 
outflow from the cold continent to the north. The south¬ 
west monsoon is much stronger than the northeast. Why? 

Similar monsoon winds are observed upon the coasts of 
South America, Africa, and Australia. 

The Temperate Zone. — Poleward from the torrid zone 
in each hemisphere lies the temperate zone. It comprises 
about one-half of the earth’s surface and shows the greatest 
variety and range of climatic conditions. Lying mainly 
under the westerlies, its weather and climate are mainly 
cyclonic in character, thus accounting for its variability of 
climate. To appreciate the climate of any part of this belt 
it is more necessary to know extremes than averages. In 
northeastern Siberia, for example, an extreme range of tem¬ 
perature during the year of over 200° F. occurs, the average 
temperature being about zero. 

In general the variability of climate in the temperate zone 
is greater over the land than over the sea, and greater in the 
northern than in the southern hemisphere. 

Divisions of the Temperate Zone. — On this account it is 
desirable to divide the temperate zones into: 

1. Ocean and land areas. 

2. Northern and southern belts. 

3. Eastern coasts, interiors, and western coasts. 

1. The ocean areas of this zone have smaller range of tem¬ 
perature, more rainfall and cloudiness, and stronger winds 


WEATHER AND CLIMATE 


191 


than the land areas. The oceans, being low-pressure areas 
in winter, have their greatest rainfall at that season; whereas 
the continents, on the whole, receive their greatest amount 
of rain in summer. 

2. Because of its large water area the climate of the south 
temperate belt is most equable of all the regions in the west¬ 
erlies. The winds toward its southern edge have high veloc¬ 
ity and blow with great regularity; and westward-bound 
ships around Cape Horn are delayed by these head winds. 

The north temperate zone has a climate that has been lik¬ 
ened to “a crazy quilt of patches,’’ so changeable is it in its 
various parts. The excess of land with its varying altitudes 
in this zone breaks up whatever tendency there might be 
toward uniformity in climatic conditions. 

3. The eastern coasts , though tempered by their adjacence 
to the sea, lying to leeward of the continents, partake of the 
variability of climate characteristic of the land. The succes¬ 
sion of lows and highs that march eastward across the conti¬ 
nents carry to the eastern coasts continental conditions. 
Winds and rains are the result of passing lows and highs; 
and rainfall is slightly more abundant in winter, when the 
winds from the sea blow upon the colder land, than in 
summer. 

The climates of the eastern coasts are to a slight extent 
modified by the warm and cold ocean currents that follow 
them. The eastern coast of the United States as far north 
as Cape Cod is to some extent warmed in winter when the 
wind blows from the southeast over the warm Gulf Stream; 
and the eastern coast of North America north of Cape Cod is 
cooled in summer, in like manner, by winds from the cold 
Labrador Current. On this account we find summer resorts 
on the coast of New England and winter resorts from Florida 
northward to New York. 

In .a similar manner the Japan Current affects the climate 
of southeastern Asia, while northward from Korea the 
coast is cooled by winds from the cold Kamchatka Current. 


192 


NEW PHYSIOGRAPHY 


Ill the southern hemisphere the eastern coasts of all conti¬ 
nents are warmed by winds from warm currents. 

The interiors of the continents show greater extremes of 
temperature and rainfall than do the coasts. The summers 
are warmer and the winters colder than the latitude justi¬ 
fies. The winds and rains are cyclonic, and the rainfall is 
most abundant in summer, when the lands are great centers 
of low pressure with inflowing winds. If the region is pla¬ 
teau, it is characterized by slightly lower temperature and 
less rainfall than if it is plain. 

The western coasts have the benefit of the tempered wester¬ 
lies coming from the ocean. This gives an evenness of tem¬ 
perature throughout the year not found upon the eastern 
coasts of the temperate zone. Rain is both more abundant 
and more markedly concentrated in the winter months than 
it is upon the eastern coasts, because in winter the colder 
land chills the moist winds from the ocean. Winter fogs 
are most frequent on this coast. 

The northern and southern hemispheres differ widely in 
the climates of their western coasts as a result of ocean 
currents. Alaska and northwestern Europe are warmed 
many degrees above the normal for their latitude by winds 
from the great warm drifts in the Pacific and the Atlantic 
Oceans, while Chile, western Africa, and western Australia 
are cooled as a result of the branches along these coasts 
from the cold Antarctic Drift. 

The climatic influence of warm and cold currents is much 
greater on windward than on leeward coasts. In the wester¬ 
lies the windward coasts are the western coasts, whereas in the 
trade belts the windward coasts are the eastern coasts. Winds 
blowing over ocean currents acquire the temperatures of 
those currents which' they carry to the lands. Winds blow¬ 
ing across continents tend to carry continental conditions 
to the eastern coasts in the westerlies and to the western coasts 
in the trades. 

While the chief effect of the warm currents in the northern 


WEATHER AND CLIMATE 


193 


oceans is to temper the cold of the western coasts in high lati¬ 
tudes, a less marked but noticeable effect is to temper the heat 
of the western coasts in low latitudes, upon the return of the 
currents toward the equator. Southern California, western 
Mexico, and northwestern Africa are thus made cooler. 

Between the temperate and torrid zones lies a transitional 
zone of high pressure and descending air currents. This 
belt shifts during the year with the northward and south¬ 
ward movement of the sun. These are monsoon belts; there¬ 
fore places in them have alternately the climate of the 
trades and of the westerlies. Rainfall is scant, owing to the 
increased temperature — which is due to compression of 
the descending currents; and the winds are never strong, 
since this belt is the starting point of both the trade winds 
and the westerlies. 

In general, seasons in the temperate zone are based chiefly 
upon temperature, whereas those of the torrid are based upon 
rainfall. The transition seasons of spring and fall, though 
well marked in the middle of the temperate zone, disappear 
toward the poleward and equatorward edges. Mountain 
ranges with a north-south trend induce rainfall upon their 
western slopes, while arid or desert regions are apt to lie to 
eastward of them. This is the reverse of conditions in the 
trade-wind belts, where the eastward are the rainy and the 
westward the dry slopes. 

The Polar Cold Caps. — The polar areas, though called 
the frigid zones, are not belts, but an area about each terres¬ 
trial pole included within the summer isotherm of 50° F. 
The polar cold cap is much more extensive in the southern 
than in the northern hemisphere, possibly due to the great 
glacial ice sheet covering the Antarctic continent or to 
aphelion winter. In the Antarctic regions the boundary 
isotherm reaches in some places to latitude 55, while in the 
Arctic regions it crosses the Arctic Circle. 

These regions are rightly named frigid, the chief charac¬ 
teristic of their climate being freezing temperatures for most 


194 


NEW PHYSIOGRAPHY 


of the year. At no time is the temperature high. The lands, 
for the most part covered by ice, are frozen deserts. It is 
only where there are slopes favorably inclined to the sun that 
the snow melts and the soil is sufficiently warmed and drained 
to permit plants to grow. Even here the temperature is too 
low for trees to thrive, and mosses and lichens are the chief 
growth. Temperatures of - 83° F. have been reported. 

The winds over the polar cold caps are cyclonic, and the 
cyclones are probably driven cyclones. The winds are often 
burdened with fine dry snow, which covers all land surfaces 
for most of the year. Over the stretches of frozen plain these 
sweep with great violence, and are comparable to the bliz¬ 
zards of winter climates in temperate latitudes. 

Precipitation, which on the whole decreases toward the 
poles, is deficient here. It is mainly in the form of snow, 
there being some regions where rain probably never falls. 
Though the precipitation is light, yet it probably exceeds 
evaporation over the region, the excess being imported by 
the westerlies from lower latitudes. 

Since more snow falls than melts, the land areas of Green¬ 
land and the Antarctic continent become covered with a 
glacial ice sheet. When the glaciers reach down to the sea, 
great blocks of ice break off and float away as icebergs to 
lower latitudes. 

Continental and Marine Climates. — The interiors of all 
continents are marked by great variability of temperature. 
This variability decreases toward the coasts, being least 
upon windward coasts. In strong contrast with the climates 
of continent interiors is the climate of islands, which is prac¬ 
tically that of the surrounding sea. 

The sea is less heated and less cooled than the land under 
the same conditions of insolation for: 

1. It is a poorer absorber and poorer radiator than the land. 

2. The specific heat of water is higher than that of land. 

3. Much of the insolation absorbed by the sea is used in evapo¬ 
ration. 


WEATHER AND CLIMATE 


195 


4. The currents in the sea distribute the heat widely. 

5. There is a greater percentage of cloudiness over the sea than 
over the land. 

All of these variables combine to give to places surrounded 
or bordered by the sea peculiarly equable climates as compared 
with continent interiors in the same latitude. The one cli¬ 
mate we call marine or insular, the other continental. 

Mountain Climates. — As we ascend a mountain in any 
latitude, all the climatic elements change from those prevail¬ 
ing at the mountain’s base. The temperature, as we have 
seen, falls at the rate of about 1° F. for every 300 feet of 
ascent; the absolute humidity decreases, while the relative 
humidity generally increases up to a certain altitude, depend¬ 
ing upon the latitude, after which it also decreases; precipita¬ 
tion increases for a time, as we ascend, then gradually fails; 
and with increasing altitude the winds increase in strength 
and constancy. 

On the whole, all of the climatic elements become more 
constant with increase of altitude, and this equability is the 
most marked characteristic of mountain climates. 

The windward and leeward sides of mountains and par¬ 
ticularly of mountain ranges are likely to present very differ¬ 
ent types of climate. The windward, owing to ascending 
and cooling currents, has an excess of rainfall, while the lee¬ 
ward, with descending and warming winds, is likely to be 
dry. If in the trades, the eastern slopes receive the rainfall, 
whereas in the westerlies the eastern slopes are dry. 

The arid regions of Nevada in the United States and of 
western Argentina in South America are examples of arid 
regions to leeward of mountains in the westerlies, and the 
arid western coasts of Mexico and Peru lie to leeward of 
mountains in the trade-wind belts. The southern slopes of 
the Himalayas receive most of their rainfall while the south¬ 
west monsoon blows; and the northern slopes of the Atlas 
Mountains in northern Africa are the windward and, there¬ 
fore, the rainy slopes. 


196 


NEW PHYSIOGRAPHY 


Climates of the Past. — It is now well known that there 
have been great changes of climate during past geological 
ages. These changes have been from warm to cold or from 
cold to warm; from dry to moist or from moist to dry. 

Evidences of these changes are found in the character of 
the rocks, in the fossil marine forms, and in the fossil re¬ 
mains of land form. Glacial climates have occurred in the 
United States as far south as New York, Cincinnati, and 
St. Louis, as well as in South America, Africa, and Australia; 
and warm temperate climates in Alaska, Greenland, and 
Spitzbergen. 

Glacial to Warm. — Evidences of former glacial climates 
are recognized chiefly in the character of deposits made by 
the ice sheet. These deposits are unassorted, and the ma¬ 
terials are angular in character and often scratched. That 
left during the last glacial period, the Great Ice Age of 
North America and Europe, is a confused mixture of clay, 
sand, gravel, and bowlders, and known in the United States 
as bowlder clay. Since it was so recently formed, it is the 
surface deposit and for the most part unconsolidated. 

Similar deposits have been found in various countries and 
geological periods beneath great thicknesses of other rock, 
and consolidated. It is known as tillite, and wherever found 
is recognized as unquestioned evidence of glacial climates 
when and where it was formed. Such deposits are made in 
no other way than by ice. 

Warm to Glacial. — All theories concerning the evolution 
of the earth agree in the belief that the sea was originally 
hot, even to boiling. Its present temperature has resulted 
from cooling at the surface, which naturally occurred first in 
polar regions where the sun’s rays are most slanting. Once 
cooled, the deep waters have remained cool, as they still are 
even under the equator, in spite of surface temperatures of 
80° or higher. Only the land and surface waters now vary 
in temperature. 

Inasmuch as glacial climates have occurred in regions now 


WEATHER AND CLIMATE 197 

warm, they must have followed as well as have been fol¬ 
lowed by warm climates. 

Fossils, Marine Forms. — Reef-making corals thrive only 
in waters that never fall below 68° F. In the present seas 
they are found mostly within the tropics, being found far¬ 
thest from the equator in the North Atlantic about the 
Bermuda Islands. This is due to the great warm Gulf Stream 
that flows past these islands. 

Fossil coral reefs are found in Tennessee, New York, 
Alaska, and within the Arctic Circle, even as far north as 
81°. These periods when the seas in high latitudes were 
warm fell between the cold periods when tillite was depos¬ 
ited, thus showing an alternation of climatic conditions 
between warm and cold. 

Fossils, Land Forms. — In tropical regions where plant 
growth is not suspended by reason of the cold, trees do not 
show rings of growth like those of trees in temperate climates. 
Most of the plants from which our coal beds were formed 
were of the types now found in tropical climates. Fossil 
trees of these types as well as beds of coal have been dis¬ 
covered well within the Arctic Circle, thus pointing to a 
one-time warmer climate there. 

Salt and Gypsum. — In arid regions where lakes and seas 
have an inflow equaled or exceeded by evaporation such 
seas are salt, and beds of salt and gypsum are being depos¬ 
ited. Great Salt Lake in Utah and the Caspian and Dead 
Seas are examples of such lakes. No such deposits form 
where the rainfall is greater than loss by evaporation, lakes 
in such regions filling up and overflowing. These regions, 
now arid, have not always been so. The plain about Great 
Salt Lake is the bed of a great fresh-water lake. A change of 
climate from moist to arid occurred, probably because of the 
elevation of the land. 

In past times there have been similar changes from moist 
to arid in regions again well supplied with rainfall. De¬ 
posits of salt and gypsum are found in New York, Ohio, 


! 


198 NEW PHYSIOGRAPHY 

Michigan, Kansas, Oklahoma, and Louisiana, deeply buried 
beneath other beds of rock. 


QUESTIONS 

1. Why are weather changes more pronounced near sea level 
than at higher altitudes? Inland than near the coast? In high 
than in low latitudes? 

2. Why do cyclones endure longer at sea than on the land? 

3. Why should rainfall be more abundant behind than in front 
of the cyclone centers in the trade-wind belts? 

4. What is the direction of the wind in front of and behind storm 
centers in the trades? In the westerlies? 

5. Why are thunderstorms more frequent in summer than in 

winter? . , 

6. Why do tornadoes generally occur in the afternoon? And 
why are not people in the United States much worried by a threat¬ 
ening cloud in the north, but seriously fear one in the southwest? 

7. What is the scientific basis for the weather proverb, “Rain 

before seven, clear before eleven”? Is there any such basis for the 
proverb, “ Morning red and evening gray sends the traveler on his 
way; morning gray and evening red sends the traveler wet to bed ? 
If so, is it true in all latitudes? _ . 

8. Why do continent interiors have their greatest rainfall m 

summer? . 

9. Why is the climate of the north temperate zone more variable 

than that of the south temperate? 

10. Why do the interiors of continents show the greatest extremes 

of climate? 

11. Why do eastern coasts in the trade-wind belts have more 
abundant rainfall than western coasts in these belts, whereas the 
reverse is true in the westerlies? Find illustrations of this fact in 
all continents. 


CHAPTER XIII 

CLIMATE OF THE UNITED STATES 

Climatic Regions. — The United States is situated in the 
zone of prevailing westerlies; its climate is chiefly of cy¬ 
clonic control. However, its wide range in latitude, its great 
variation in distance from the sea, and the difference in altitude 
of the various parts give to different sections sufficiently 
characteristic climates to justify their separate consider¬ 
ation. 

Minnesota and Maine, because of their higher latitude, 
have lower average temperatures than Louisiana and Flor¬ 
ida; Kansas and Nebraska, lying near the center of the con¬ 
tinent, have greater ranges of temperature and less rainfall 
than northern California and Maryland; and Denver, in 
the foothills of the mountains, has a more equable climate 
than St. Louis, about the same latitude but at a lower level. 

Based upon these three conditions governing climate 
— latitude, distance from the sea, and altitude — it has 
been suggested that the United States be divided into cli¬ 
matic regions. Some of these regions vary considerably from 
north to south. 

The Pacific Coast Region. — This zone extends inland 
from the Pacific coast about 200 miles to the backbone of 
the Sierra Nevada and Cascade Mountains. Like all regions 
situated to leeward of an ocean, it is characterized through¬ 
out by an equable temperature. The isotherms, instead of 
having a roughly east-west trend, as is their usual habit, 
run almost parallel with the coast. The continuation of the 
Japan Current, the North Pacific Drift, cooled during its 

199 


200 


NEW PHYSIOGRAPHY 


long journey through North Pacific waters, in its southward 
flow washes the entire length of coast of this region. The 
influence of the winds from over this current is perhaps to 
increase the temperature of the state of Washington slightly 
above the average for that latitude, but to lower the tem¬ 
perature of southern California decidedly. Frost seldom 
occurs here in the lowlands. 

In strong contrast with this sameness of temperature 
throughout its north-south extent is its wide difference in 
annual rainfall. The westerly winds come from the Pacific, 
moisture laden at all seasons. In summer they blow upon 
lowlands warmer than themselves and, therefore, yield no 
rain until they begin to rise up the mountain slopes. In 
winter the cooler land induces rainfall, even over the low¬ 
lands, thus making the winter rains the most marked charac¬ 
teristic of the Pacific coast climate. 

Upon the mountain slopes the rainfall is abundant through¬ 
out the year, though most abundant in winter. There we 
find the giant trees and dense forests. On the coast of 
Washington, where the high mountains lie near the sea, we 
find the greatest rainfall of the United States, more than 
100 inches, whereas in southern California, with its coastal 
plain, and its nearness to the high-pressure calms, it is less 
than 10 inches. 

The cultivated lowlands at the south are parched during 
the growing season; and but for their nearness to the moun¬ 
tains, which makes irrigation possible, these lowlands would 
be of little value. As it is, they are among the most valuable 
cultivated lands in the United States. 

It is maintained for these lands that they are peculiarly 
adapted to the production of fruits, inasmuch as the fruits 
grow and ripen in sunshine , thus giving them higher color 
and superior flavor. 

Along the coast, fogs are common, especially in winter. 
Severe storms are almost unknown, thunder being rarely 
heard upon the coast. Upon the mountain slopes thunder- 


CLIMATE OF THE UNITED STATES 


201 


storms break, and the lightning flashes are seen, though at 
distances from the coast too great for the sound of the 
accompanying thunder to carry. 

The Interior Basin. — This region embraces the high 
lands lying between the Sierra Nevada and Cascade Moun¬ 
tains on the west and the Rocky Mountains on the east. 
Its most marked characteristic is its dryness. Lying as it 
does to the leeward of the Sierras and Cascades, the descend¬ 
ing winds on the eastern slopes of these mountains yield 
little rain. It is only after they have crossed the greater 
part of the region and begin their ascent of the western 
slopes of the Rockies that rain is induced. Occasional cy¬ 
clonic storms yield some rain, but over much of the region 
the rainfall is insufficient for agriculture, without irrigation. 
It varies from 20 inches in Washington to three inches in 
Arizona. A part of this region is too remote from the moun¬ 
tains to permit of irrigation and must, therefore, remain arid 
and unproductive. 

The skies over the interior basin are prevailingly clear, 
consequently the daily range of temperature is excessive. 
The winters are cold, and the .summer days extremely hot. 
Cold winter cyclones sweep down from the north, and it has 
been suggested that the hot desert region about the head of 
the Gulf of California is the birthplace of most of our south¬ 
west summer cyclones. 

Rainfall is nowhere in the region sufficient to support 
heavy forests. At the north, where most abundant, it falls 
mostly in winter, the growing season being almost without 
rain. Owing to the deep and retentive soil, so fine-grained 
and homogeneous that it brings capillary water from unusual 
depths, this part of the region yields abundant wheat har¬ 
vests. Apples and other fruits of temperate latitude are 
grown where irrigation is possible. 

The chinook winds, which sink down the mountain slopes, 
warming as they advance, keep the narrow mountain val¬ 
leys free from snow. On this account these valleys are much 


202 


NEW PHYSIOGRAPHY 


sought by both wild and domestic animals for winter grazing 
grounds. 

The Great Plains. — This name is applied to the region 
of eastward sloping lands from the Rocky Mountains to 
about the meridian of 100° west. It grades imperceptibly 
eastward into the next climatic region. Most of the region 
is characterized by the cold winters and hot summers, typi¬ 
cal of continental interiors in this latitude. 

Rainfall, which increases eastward with increasing dis¬ 
tance from the mountains, is in the main insufficient for 
agriculture, unaided by irrigation. Much of the region is 
capable of irrigation from streams or artesian wells, and by 
this means the land is becoming increasingly valuable. The 
rainfall is insufficient for forests, but it suffices for the growth 
of abundant and nutritious grasses. These are the great 
natural grazing grounds of the United States. Before the 
advent of the white man herds of buffalo roamed these 
plains, but disappeared with the march of civilization west¬ 
ward. In their stead came herds of cattle and flocks of 
sheep, and that typically western product, the cowboy. 

The seasons are variable in the extreme. Occasional abun¬ 
dant harvests are gathered, only to be followed by one or 
more seasons of disastrous failure. With wider adoption of 
the methods of “ dryfarming f ’ much more of the Great 
Plains region will be devoted to agriculture. 

The rains and winds of the region are wholly determined 
by the passage of highs and lows. The rainfall is distributed 
through the year, though slightly in excess in summer, and 
nowhere exceeds 20 inches. 

This region is the continuation of the great Arctic plain, 
which extends unbroken southward past Hudson Bay. With 
no east-west mountain range to interrupt, it is swept over 
by the winter cyclones from the north, which sometimes 
reach even to the Gulf of Mexico before turning to their 
final northeastward course. Owing to the level and prairie 
character of the region, wind velocities are often excessive. 


CLIMATE OF THE UNITED STATES 


203 


The Central Prairie Lands. — As already stated, this 
region is a continuation eastward of the Plains region, there 
being no natural boundary between them. It is bounded 
eastward by the Mississippi River. 

The Central Prairie Lands differ from the Great Plains 
chiefly in having a more abundant rainfall, 20 inches or 
more. This is everywhere sufficient for agriculture, and in¬ 
creases southward. On the coast of Louisiana it is 60 inches, 
due to the indraft of warm winds from the Gulf, toward 
lows crossing the region farther north. 

The climate, though typically cyclonic, is not so extreme 
as farther west. At the south the influence of the Gulf in 
tempering both the cold of winter and the heat of summer 
is marked. The annual range of temperature is 160° in 
North Dakota, whereas it is but half that on the Gulf coast. 

Over most of this region the rainfall, 30 inches, is sufficient 
to support forests, and their absence over much of the region 
has not been satisfactorily explained. Forests border prac¬ 
tically every stream of the region. 

Various explanations of the absence of forests in this 
region have been proposed. The one which perhaps has 
received widest acceptance is the destruction of the forests 
by fires. Inasmuch as attempts to extend the forests have 
not been successful, it would seem that perhaps the explana¬ 
tion of the absence of forests is to be found in the character 
of the soil and of deeper deposits, which are often glacial clays. 

The great body of the more productive agricultural lands 
lies in this division, and here most of the staple food prod¬ 
ucts are grown. 

The winds are variable in direction, though northerly 
winds prevail at the north and southerly winds prevail at 
the south. Owing to the greater interruption by forests, the 
winds do not here attain the average strength of the winds 
upon the Great Plains. 

This climatic region is visited by a greater number of de¬ 
structive windstorms than any other region of the United 


204 


NEW PHYSIOGRAPHY 


States. Tornadoes begin to occur in the Gulf States in Feb¬ 
ruary, though most frequent from April to September. 
Their time of earliest occurrence is later the farther north 
we go. 

The Western Appalachian Slope. — This region embraces 
the area extending from the Mississippi eastward to the axis 
of the Appalachian Mountains, and from the Great Lakes 
southward to Tennessee. At the north, the Great Lakes 
temper both the heat of summer and the cold of winter, so 
that the climate, though continental, is not so extreme as 
in the regions between the Mississippi River and the Rocky 
Mountains. 

The westward trend of the Appalachians, while protect¬ 
ing the Gulf slopes to the southward from cyclones originat¬ 
ing in the west, also protects the climatic region to the 
northward from the frequent tropical storms that come up 
from the West Indies. These tropical cyclones rarely cross 
the mountain barrier of the Appalachians. 

The rainfall of the region exceeds that of the northern 
division of the prairies for two reasons: there is a greater 
water surface adjacent to yield vapor and the prevailing 
westerlies are moving up the slopes. The rivers are there¬ 
fore numerous and strong and more evenly and abundantly 
supplied than are those of the prairie region. Though well 
distributed through the year, the rainfall is more abundant 
in summer than in winter. This is in part due to the greater 
absolute humidity of the air in summer, and in part to the 
more frequent passage of lows having their origin in the 
southwestern part of the United States. Winter cyclones 
more commonly originate in the northwest and are not so 
likely to be accompanied by precipitation. The precipita¬ 
tion in winter is chiefly in the form of snow, especially in 
the lake region. 

The winds are cyclonic, but less strong than in the prairie 
region, because of the generally forested character and 
greater unevenness of the lands of this region. 


CLIMATE OF THE UNITED STATES 205 

The Atlantic-Gulf Slope. — This slope, extending from 
Maine to Louisiana, presents a great variety of climate. 
Since it is near the sea, neither the extreme cold of winter nor 
the heat of summer of regions farther inland is felt; but 
being to leeward of the continent, the equalizing influence of 
the sea is not nearly so great as upon the Pacific slope. At 
the north the winters are cold and the summers cool; while 
at the south the winters are temperate, and the summers, 
owing to the excessive humidity, are oppressive, though not 
so warm as farther north inland. 

Ocean currents are important factors in determining the 
climate of the Atlantic coast. The cold Labrador Current, 
coming down the New England coast, makes that coast 
colder as far south as Cape Cod, while the Gulf Stream in¬ 
fluences the climate of the coast from Florida northward 
to Cape Cod. 

Rainfall, abundant throughout this climatic region, in¬ 
creases generally toward the south, where it is more than 
60 inches. It is well distributed throughout the year, though 
for the greater part of the region it is most abundant in the 
autumn. Toward the south the maximum fall is later, in 
southern Florida being most abundant in winter when the 
westerlies prevail. However, the southern Florida rains are 
not of the Pacific coast type of winter rains, being mainly 
due to passing lows, and not to forced ascent over cold lands. 

South of Cape Hatteras the coast is often swept by tropi¬ 
cal cyclones which reach our coast from the southeast, 
whereas north of Hatteras the cyclones are chiefly from the 
west or southwest and originate outside the tropics. Both 
types of storms move northeastward, their paths converg¬ 
ing, thus giving to New York and Boston a greater number 
of cyclonic storms than to points either farther north or 
farther south. 

Exceptional Conditions. — In many of his activities man 
is controlled not so much by usual as by exceptional condi¬ 
tions of climate. Our buildings are constructed to with- 


206 


NEW PHYSIOGRAPHY 


stand the strongest wind and the levees along our rivers to 
restrain the highest flood. 

As we have seen, profitable agriculture is not so much de¬ 
pendent upon the annual rainfall as upon the occurrence of 
rain during the growing season. 

In order to obtain a clear understanding of the climates of 
the several climatic regions of the United States, it is neces¬ 
sary to examine maps showing averages for given periods and 
maps showing departures from these averages. 

From the January chart of temperatures one can see the wide 
difference in the winter temperature of 70° at Key West and of 
- 5° in North Dakota. This difference, while in part due to dif¬ 
ference of latitude, is to a much greater extent the result of the 
difference between coast and continent-interior conditions. Along 
the Atlantic coast the change in temperature is from 70° at Key 
West to 10° in northern Maine, while along the Pacific coast, for 
the same change in latitude, there is a change of only 10° in tempera¬ 
ture. The temperature contrast shown by these two coasts illus¬ 
trates a difference due mainly to position to leeward of a continent 
and position to leeward of an ocean. 

The July chart of temperatures tells a very different story. No 
longer is the highest temperature found at Key West, but in Arizona, 
some 8° farther north. This is largely due the arid character of this 
region, with its prevailingly clear skies. Along the Atlantic slope 
there is a difference of temperature of about 25° between Florida 
and Maine, as compared with 60° in January, while on the Pacific 
coast the difference for July is about the same as for January, the 
isotherms, as we see, running almost parallel with the coast. The 
interior of the continent, which is colder than similar latitudes upon 
the coast in January, is now seen to be warmer. The isotherms for 
July bend northward in crossing the continent, whereas those for 
January bend southward. 

While the lines of equal minimum temperature follow in general 
the trend of the isotherms for January, the lines of equal maximum 
temperature are not so regular. We find the lowest minimum, — 63°, 
in the interior of the continent near its northern boundary and the 
highest minimum, 40°, at Key West. Here frost never occurs. The 
minimum temperatures of the Atlantic coast are from 20° to 30° 


CLIMATE OF THE UNITED STATES 207 

lower than those of the Pacific coast in the same latitudes. Here 
again is shown the difference in windward and leeward coasts. 

The tempering influence of the sea is well shown by a comparison 
of the average maximum of the coasts, about 95° F.,‘with that of 
the interior, which is about 10° higher. The lowest maxima, about 
90°, are found in the extreme northeast and northwest coastal re¬ 
gions, while the highest, almost 125°, occur in the interior desert 
region of southern California and Arizona. Maxima exceeding 105° 
are common over the Great Plains region, but the dry heat of this 
region is not so oppressive as that of the Gulf and Atlantic coasts, 
where the maxima are 5° lower. 

The range of temperature is the difference between the summer 
maximum and the winter minimum. The greatest range is found in 
the northern interior, whereas the lowest range occurs at Key West. 
In general, range of temperature increases with increase of latitude 
and with distance from the sea . 

The range of temperature along the Pacific coast varies little, 
being only 15° greater at Puget Sound than in southern California, 
whereas the Atlantic coast varies in range from 50° in the south to 
110° in Maine. The range of temperature for most of the Gulf 
coast is about 85°, whereas that for Montana is twice as great. 

Freezing Temperatures. — The number of days with average tem¬ 
perature below freezing varies from none in the Pacific, Gulf, and 
Atlantic coast regions northward to Chesapeake Bay, to 165 days 
in Minnesota and North Dakota. Of much greater practical interest 
to farmers and fruit growers, however, are the dates of occurrence 
of earliest and latest killing frosts. 

In the fall, with the lengthening night and increasing slant of 
the sun’s rays, there comes a time when the daily minimum falls 
almost to freezing. The passage of a low across the continent then 
is likely to be followed by frosts. These are due to the cold indraft 
of north winds at the rear of the low, where the sky is clear and the 
winds light. 

The date of occurrence of the first killing frost in the extreme 
north central part of the United States is about the first of Sep¬ 
tember. As the winter season marches southward, and toward the 
coasts, the first killing frosts occur later and later in these directions, 
being as late as December 15 in central Forida. 

In spring, when the noon altitude of the sun is increasing and the 


208 


NEW PHYSIOGRAPHY 







. .i ..... 

w 

i 

* 

■ V xS* 

1 

5 

A 

WM 


Fig. 94. — Isotherms of the United States for January (After Henry) 





























CLIMATE OF THE UNITED STATES 


209 



Fig. 95 . — Isotherms of the United States for July 















210 


NEW PHYSIOGRAPHY 



frost. Account for this freedom. (Henry.) 









































































CLIMATE OF THE UNITED STATES 


211 



Fig. 97 . — Showing Date op Latest Killing Frost in Spring 
Spring marches north-westward across the country. (Henry.) 








































212 


NEW PHYSIOGRAPHY 


days are lengthening, there comes a time when the ordinary daily 
minimum ceases to fall below freezing. But for weeks after this 
condition exists, a passing low, with its cold in-draft of northern 
winds behind, may bring freezing temperatures; and thus the time 
of latest killing frost be made later. At such times falling tempera¬ 
ture and clearing skies forewarn of frost. 

Since spring marches northward and landward from the coasts, 
the average time of latest killing frosts is earliest at the south; and 
in any latitude, at the coast. It occurs along most of the Gulf 
coast about the first of February and is delayed in the extreme 
northern part of central United States until almost the first of 
June. 

The absolute date of latest killing frosts is considerably later than 
the average date in all sections, being much nearer March 1 on the 
Gulf coast, and July 1 in Minnesota. 

Distribution of Rainfall in United States. — From the accom¬ 
panying rainfall charts we are able to locate the regions of greatest 
and of least rainfall during the year as well as the more important 
matter of its distribution in time. For the farmer and planter this 
last is of the greatest importance. 

The least rainfall, three inches, occurs in southwestern Arizona. 
Most of this amount may fall in a single day, or indeed in a few 
hours, during a single thunderstorm. The greatest annual rainfall 
in the United States, more than 100 inches, occurs in northwest 
Washington, and while most abundant in winter, is fairly well dis¬ 
tributed through the year. The annual rainfall on the Pacific c’oast 
decreases southward, in central California being but half of the 
maximum in Washington. 

On the Atlantic coast the maximum rainfall is near Cape Hat- 
teras, decreasing northward and southward. 

A rainfall of two to four inches a month during the growing 
season is desirable for agriculture. Many times this amount falls, as 
much as 10 inches being recorded in a single day. Such torrential 
downpours are injurious alike to growing crops and to cultivated 
lands. The soil is washed away; streams are flooded and overflow 
their banks, causing destruction of property and life. These heavy 
downpours are popularly known as cloudbursts. 

The recorded rainfall includes snowfall, 10 inches of snowfall being 
estimated, when melted, as the equivalent of one inch of rain. 


Fig. 98. Average Rainfall of United States 
United States Weather Bureau. 


CLIMATE OF THE UNITED STATES 


213 



























































214 


NEW PHYSIOGRAPHY 


Distribution of Snow. — Every part of the United States, ex¬ 
cepting southern Florida and southern California, receives some 
snowfall. It is least at the south, and increases with latitude and 
altitude. It is more than 40 inches in the region of the Great Lakes 
and in the Rocky Mountains, and occasional heavy snowfalls occur 
in the extreme south. A fall of 13 inches occurred at Baton Rouge, 
Louisiana, during a single storm in February; but such snows 
usually melt within a day or two after falling. 

The greatest annual snowfall in the lowlands of the United States, 
130 inches, occurs in the northern peninsula of Michigan, the mois¬ 
ture being supplied from the adjacent lakes. The greatest average 
annual snowfall of the entire country, not including Alaska, occurs 
in the Sierra Nevada Mountains. The moist westerlies from the 
Pacific, compelled to rise in passing over the mountains, precipitate, 
on an average, 378 inches of snow at Summit, California. 

The Rocky Mountain region has a heavy annual snowfall, though 
less than the Sierra Nevada and Coast ranges. It is mainly to the 
melting of these snows in the Rockies that the great irrigation proj¬ 
ects look for their supply of water. The floods in the Missouri and 
other eastward-flowing streams with sources in these mountains 
occur in May and June, when the normal rainfall is augmented by 
the melting snow. 

The snowfall in the northern plains and prairie regions is variable. 
Some winters it is excessive; others, light. When abundant in the 
wheat-growing sections a good crop is expected, since the snow 
serves as a protection from the cold, and also leaves the soil in 
good condition. 

In the lumbering sections of the north, from Minnesota to Maine, 
the profits of the season are directly related to the snowfall, which 
is usually abundant. Little snowfall in these regions means smaller 
output. 

Number of Days with Precipitation.—The number of rainy or 
snowy days during the year varies widely in different sections of 
the country. In general, it is least in the interior, and increases 
toward the coasts; and is greater in the north than in the south. 
The greatest number, 180, occurs in northwest Washington; then 
follows the Great Lakes region with 170 days. In the southwest 
desert region the number falls to 13. For most of the agricultural 
sections the number varies from 100 to 140. Forty consecutive rainy 


Fig. 98A. — Types of Rainfall Distribution in the United States (Henry) 


CLIMATE OF THE UNITED STATES 


215 



Rainfall Distribution in- the U. S. Percentage of fall in each month represented by heavy vertical lines. 




























































































































































































































































































216 


NEW PHYSIOGRAPHY 


days are reported in northwestern United States, and 150 days of 
consecutive drought in the arid region of the southwest. 

Humidity. —- The absolute humidity of the air is greater in south¬ 
ern United States than in northern; it is greater in summer than in 
winter and greater near the coast than in the interior. 

The relative humidity is on an average lowest on the Colorado 
plateau, where it is about 40 per cent, and highest on the eastern and 
western coasts, about latitude 40° N., where it is about 80 per cent. 
In the Gulf region it is about 75 per cent, and approximately the 
same in the region of the Great Lakes, although, as a rule, conti¬ 
nental interiors favor low relative humidities. 

The percentage of cloudiness agrees well in winter with the rela¬ 
tive humidity, but in summer one of the areas of greatest cloudiness 
is over the Colorado plateau, where the average relative humidity 
is low. This is probably due to the strong convectional currents 
set up during the summer season, the air rising to sufficient heights 
for saturation. 

Winds. — As before stated, the winds are stronger upon 
the coast and over the prairie regions than over forests and 
mountainous regions. For most of the country the season 
of strongest winds is spring, and the month of weakest winds 
is August. 

Aside from tornadoes and hurricanes, during which, for 
a few seconds, the velocity of the wind may considerably 
exceed 100 miles an hour, the strongest winds are about 70 
miles an hour inland and 90 miles an hour on the coast. 

Though the direction of the wind is variable in all parts 
of the United States, in valleys there is a decided up- or down- 
the-valley tendency in wind direction. On the coasts, in 
winter, there is a predominance of land winds. This is es¬ 
pecially true of the Gulf and Atlantic coasts. On the Pacific 
coast the meeting, of the land winds and the prevailing west¬ 
erlies produces “ along-shore ” winds. In summer the con¬ 
ditions upon the Atlantic and Pacific coasts are reversed. 
The Pacific now has strong ocean winds, while the in-blowing 
winds upon the Atlantic coast are met by the westerlies, pro¬ 
ducing “ along-shore ” winds from the southwest. 


CLIMATE OF THE UNITED STATES 


217 


The winds which bring cloudy weather and precipitation 
vary with the section. They are generally winds blowing 
from the nearest great water body. On the Atlantic and 
Gulf coasts and over most of the interior of the United 
States, they are east and southeast winds, while on the Pa¬ 
cific coast they are generally southwest winds. In winter in 
the northern section of the United States, snow often accom¬ 
panies winds from a northerly direction. Winds from south¬ 
erly directions, in front of the low, bring higher temperatures 
and yield rain, while the colder winds in the rear yield snow. 

QUESTIONS 

1. Why is the rainfall of the Pacific coast so much greater in 
Washington than in southern California? And why are the rains 
at the north less distinctly winter rains than those at the south? 

2. Why should thunderstorms be practically unknown upon the 
Pacific coast? 

3. Are the Sierra Nevada or the Pocky Mountain ranges more 
responsible for the arid climate of the Great Basin? Why? 

4. Why does the Atlantic coast have so much greater variation 
in temperature than the Pacific? 

5. From what direction do storms in your section usually come? 

6. What direction of wind is usually coldest? 

7. What direction of wind is most apt to bring snow in winter 
and rain in summer? 

8. How does knowledge of your climate concern your daily life 
and occupation? 

9. Why do the isotherms upon the Pacific coast so nearly parallel 
the coast? Why is the tendency to parallelism to the coast less 
pronounced upon the Atlantic coast? 

10. What is the date of the earliest killing frost in your section? 
Latest killing frost? 

11. About what is the highest temperature of summer in your 
section? Lowest temperature of winter? 

12. About what is your yearly rainfall? 































































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PART III 


THE SEA 

















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CHAPTER XIV 

GENERAL CHARACTERISTICS OF THE SEA 

The Relation of the Sea to the Land. — Most of the phe¬ 
nomena connected with the wearing away of the land, with 
moderating the climate, and even with the existence of life 
itself, depend in large measure upon the sea. The source of 
the water supply for the land is the sea and the streams 
with their sediments from the land return to it. 

The sea is a great international highway and plays an im¬ 
portant part in the commerce of the world. It is no longer a 
barrier between countries. The great steamships are little 
affected by storms at sea. Equipped with wireless tele¬ 
graph instruments, ships communicate with each other at 
sea and with land stations, thus removing the isolation that 
was formerly experienced in crossing the great oceans. 
Countries are connected by submarine cables and radio so 
that news is sent and business transacted between nations 
separated by oceans almost as easily as between different 
parts of the same country. The digging of canals across 
isthmuses tends to change routes of travel and commerce at 
sea. The Suez Canal has had a far-reaching effect in trade 
in the Old World, and the Panama Canal has changed 
trade routes in the New. 

The surface of the sea is commonly regarded as having a 
very nearly uniform level, known as the “ level of the sea,” 
from which land elevations and sea depressions are meas¬ 
ured. The sea is drawn toward and upon the continents 
that surround it, especially when large mountain masses 
are situated near the coast, so that sea level cannot be of 
uniform curvature. The actual deformation of the ocean 

221 


222 


NEW PHYSIOGRAPHY 


level in different parts of the earth due to this cause has 
been estimated to amount to several hundred feet. On the 
coast of India, owing to the attraction of the great Himalaya 
Mountains, the water stands much higher than water in 
midocean or water along a lowland coast, such as western 
Europe or that of the eastern United States. 

The extent of the sea has not been constant in ages past 
and is now a fixed area. Much of the land furnishes evi¬ 
dence that it has at some time been covered by the sea, and 
regions now sea bottom have been land. The great central 
valley of the United States was once sea floor, there being 
an unbroken stretch of sea from the Gulf of Mexico to the 
Arctic Ocean. On the other hand, land along the eastern 
coast of North America has suffered drowning. 

Scientific explorations of the sea, made by different gov¬ 
ernments, by societies, and by individuals, from time to 
time, have given us most of our knowledge of the depth of 
the ocean, its temperature, its movements, its deposits, and 
its life. 

Divisions of the Sea. — The continuous body of salt water 
called the sea, covering about three-fourths of the earth’s 
surface, has five divisions, called oceans. The polar circles, 
the continents, and the meridians from their southern points 
form the boundaries. 

The Pacific is the largest ocean, comprising three-eighths of 
the entire sea area. Its greatest width is about 10,000 
miles, in a direction east and west along the equator. It is 
characterized on its Asiatic shores by numerous border 
seas, festoons of islands, and many rivers; and on its Ameri¬ 
can shores by high mountain ranges parallel to the shore and 
few rivers. 

The Atlantic is the second in size, with an area about one- 
quarter of the whole sea surface. It has an average width 
of 3600 miles. The equator divides both the Atlantic and 
the Pacific Ocean into a northern and southern part. The 
North Atlantic, both on the American and the European 


GENERAL CHARACTERISTICS OF THE SEA 223 

sides, has many seas and bays which give it an irregular 
shore line. It has a wide continental shelf and many rivers. 
The South Atlantic has a more even shore line and few 
good harbors. 

The Indian Ocean has an outline that is roughly circular. 
It has one-eighth of the total sea area and a diameter of 
about 6000 miles. The Indian Ocean is bordered by large 
seas and bays and a northern and western boundary con¬ 
sisting of very high plateaus and mountains. 

The Arctic Ocean is an extension of the Atlantic. It has a 
width of about 2500 miles and about one-thirtieth of the sea 
area. A considerable area of the Arctic is covered most of 
the year with drifting ice. The water at the center, in the 
vicinity of the north pole, is nearly two miles deep. 

The Antarctic Ocean lies within the Antarctic Circle. 
Within this region there is a continent covered with an ice 
cap thousands of feet thick. The relative area of land and 
water in this frozen region is at present unknown. 

The south pole is located on land with an elevation 
approaching two miles. The continental ice sheet, thousands 
of feet thick, is the source of the icebergs of the southern 
seas just as the glacier covering Greenland furnishes the 
icebergs of the North Atlantic. 

Distribution of the Ocean Waters. — On a globe held so 
that the greatest expanse of water is visible, the island of 
New Zealand will appear close to the center of the water 
hemisphere, or what might be called the water pole of the 
earth. London, England, will be nearly opposite, the center 
or pole of the land hemisphere. 

Depth. — The greatest known depth of any ocean, over 
34,000 feet, is in the Pacific near Japan. This depth is 
about a mile greater than the height of the highest moun¬ 
tain above the sea level. Many places in the sea are 
more than four miles deep, and the area of surfaces of the 
sea floor in deep water greatly exceeds the area of high 
land. The average depth of the ocean is about 2| miles, and 


I 


224 


NEW PHYSIOGRAPHY 



Fig. 99. — Chart of the Oceans, Showing the “Deeps 

After Sir John Murray. 














































































































GENERAL CHARACTERISTICS OF THE SEA 225 


the average height of land about half a mile. It may be 
inferred from this that the continental land masses would 
make a small beginning in filling up the deep sea. 

Composition of Sea Water. — The water of the sea is so 
salt and bitter as to be undrinkable. If 100 pounds of sea 
water are evaporated, about 3| pounds of a whitish powder 
will remain. About three-fourths of this powder is common 
salt. The bitterness is due to chloride of magnesia, Epsom 
salts, gypsum, and small quantities of other soluble sub¬ 
stances. Sea water contains in addition to mineral matter 
dissolved atmospheric gases. Oxygen is more abundant in 
the water near the surface, and the proportion of carbon 
dioxide increases toward the bottom. The oxygen dissolved 
in the water is being consumed by marine life, and its 
supply is furnished by the atmosphere. The amount of 
saltness of the sea varies slightly in different parts of the 
earth. Where evaporation is more rapid, as in the trade- 
wind belts, the saltness of the water is greater, since salts 
are left behind when sea water evaporates. When rainfall is 
abundant, as in the doldrum belt, the sea water becomes less 
salt and of less density. Rivers bring fresh water to the sea, 
which mixes with the salt water and makes it of less density. 

Temperature. — The surface waters of the sea are warmest, 
because the water is heated by the sun’s rays; and the 
warmer water, since it is lighter than the colder water, re¬ 
mains at or near the surface. The temperature varies from 
about 80° near the equator to about 29° near the poles. The 
decrease of temperature with increase of latitude is far from 
being regular, the irregularity being largely due to ocean 
currents which vary in temperature from that of the sur¬ 
rounding water. 

The surface waters of the sea are alternately warmed and 
cooled in both hemispheres, depending upon the season of the 
year. At the equator and the poles the seasonal change is 
slight, but in middle latitudes it amounts to several degrees. 
In the latitude of New York the winter temperatures are 


226 


NEW PHYSIOGRAPHY 


usually between 50° and 60°, and the summer temperatures 
between 60° and 70°. 

The temperature of water below the surface falls rapidly 
with increase of depth. Even near the equator the tempera¬ 
ture at a depth of less than half a mile is usually below 40°. 
At the bottom of the deep sea the temperature is generally 
below 35°. 

The decrease of temperature with increase of depth is not 
uniform because of the deep circulation of the ocean water. 
Because of currents beneath the surface, sometimes warmer 
and sometimes colder, slight irregularities in temperature 
occur. Sea water, when cooled either by cold air or by 
melting ice, tends to sink. The great supply of cold water 
from the polar regions creeps along the bottom of the sea and 
is the cause of the low temperature in the equatorial as well 
as in the temperate and polar regions. The temperature of 
the deep water in enclosed portions of the sea, such as the 
Mediterranean, in low latitudes, never falls to the low 
temperature of the deep open sea because of the raised sea 
bottom in the straits, which acts as a barrier and keeps out 
the creep of cold water. 

Sounding and Dredging. — The depth of the ocean water 
and the nature of its bottom are studied both for economic 
and scientific reasons. Before submarine cables are laid, 
suitable routes must be determined. 

Soundings of the deep sea are made by means of a weighted 
wire. The weight, called the sounding lead, surrounds a 
metal tube and is attached in such a way that when the tube 
strikes bottom the weight is released and remains on the 
bottom. The tube has a device for bringing up specimens of 
material found on the sea bottom. At intervals along the 
sounding wire, specially devised minimum thermometers 
are attached, which record the temperature at the various 
depths reached. It will be seen that by a single sounding 
not only are depths measured, but temperatures at different 
depths and a sample of deep sea deposit are obtained. 


GENERAL CHARACTERISTICS OF THE SEA 227 

By dredging, specimens of deep sea life are obtained. A 
basket of lai ge dimensions and with a flaring opening is 
dragged along the ocean bottom, and various remains and 
forms of animal life are brought to the surface. 

The ocean floor has its mountain ranges, its plateaus, and 
its plains. There are great volcanic peaks in many places, 
some of which rise higher above the sea bottom than any 
mountain of the land rises above the platform on which it 
rests. Dolphin Ridge is a broad area in mid-Atlantic over 
which the depth varies from 5000 to 12,000 feet, and is 
bordered on either side by the relatively steep slopes of 
great troughs in which the water is from 15,000 to 25,000 
feet deep. Chains of islands like Cuba and its neighbors 


Continent 



Edge of Shelf 


Ocean Basin 


----I-’ '' - 

Fig. 100. — Section Showing Continental Shelf, AB 


are believed to be the peaks of submerged mountain ranges. 
In these major features the ocean floor resembles the land. 

The most striking characteristics of the ocean bottom are 
the smoothness and the absence of the steep slopes so familiar 
on land. Below sea level the slopes of volcanoes and the 
“abrupt” slope at the outer margin of the continental 
masses are rarely steeper than a rise of one foot in twenty. 

There are very few slopes on the ocean floor that would be 
considered difficult for an automobile to climb or that are 
steeper than some of the grades on our trunk line railways. 

The smoothness of the ocean floor is due largely to the 
absence of those agents of erosion which sculpture the land 
into hills and valleys. It is due also to the accumulation 
of deposits in depressions. Between the shore line and the 
seaward limit of wave action, waves and shore currents are 









228 NEW PHYSIOGRAPHY 

spreading out land sediments, forming a smooth and nearly 
level area. Beyond this area deposits of several kinds are 
constantly accumulating, and as the deep water here is 


Fia. 101 — The Continental Shelf of North America 
After a model by Howell 

practically at rest, the sediments settle, filling depressions 
and maintaining a nearly level surface. 

It is interesting to study the way chalk settles from a 
mixture of prepared chalk and water. This mixture is 
somewhat similar to some of the oozes which settle on the 



GENERAL CHARACTERISTICS OF THE SEA 229 


ocean floor. We notice that the surface of the sediments is 
more nearly horizontal and more regular than that of the 
bottom of the vessel. This sort of action is continually, 
though slowly, in progress on the ocean floor, which is 
gradually approaching a level surface. 

The Continental Shelf. — Near the borders of the conti¬ 
nents the sediments brought down by streams, and materials 
worn from the land by the waves, are spread out by the waves 
and currents, forming a gently sloping smooth floor which is 
called the continental shelf. The continental shelf is, strictly 
speaking, a portion of the continental mass rather than a 
portion of the ocean basin. It extends seaward to the 100- 
fathom line, where the slope, becoming steeper, descends to 
the bottom of the ocean basin proper. 

The continental shelf is well developed along the eastern 
coast of North and of South America, in places more than 
100 miles wide. On the western coast it is in most places 
much narrower. The British Isles are on the continental 
shelf that borders northern Europe. 

There is evidence that much of the area of the continental 
shelf has been above sea level. Several of the valleys of 
large rivers flowing into the Atlantic may be traced seaward 
across the continental shelf by valleys or canyons which 
were corroded by the river when the continental shelf was a 
part of the dry land. 

Although about 14 per cent of the area of the ocean waters 
at the present time lie on the continental shelf, it is, strictly 
speaking, a platform of the continental mass rather than a 
part of the ocean basins. 

The ocean waters more than fill the great ocean basins and 
consequently cover the edges of the continents. 

Materials of the Ocean Floor. — The ocean is the great 
settling basin of the world. The rivers are constantly bring¬ 
ing in vast quantities of sediment and lesser quantities of 
dissolved mineral. Waves cut into the land and add much 
to the contribution of the streams, and a considerable quan- 


230 


NEW PHYSIOGRAPHY 


tity is added by the winds. The solid matter thus received 
is assorted, transported, and deposited in beds, which may 
ultimately become sedimentary rocks. A large part of the 
dissolved carbonates is taken up by plants and animals 
which change it to some such solid form as coral or shell which 
is eventually added to the deposits of the ocean floor. 

Deposits of the Continental Shelf. — These consist of 
sand and gravel beds and mud beds. Gravel beds are usually 
found near the mouths of rivers or in localities where the 
wave action is particularly violent. Sand beds sometimes 
extend many miles from the shore. The mud beds are made 
up of the finest particles and are located beyond the sand 
in the open sea or in the quiet water of bays. Pure limestones 
are formed in clear water beyond the mud beds. The de¬ 
posits on the continental shelf grade into each other. 

Deposits of the Deeper Ocean. — Beyond the mud de¬ 
posits, the only material derived directly from the land 
which accumulates on the ocean bed is the dust from the 
air; and this is so small in amount that it is overshadowed 
by the organic remains. The waste materials of the land 
extend some distance beyond a depth of 600 feet, but they 
gradually disappear and are replaced by oozes which cover 
the bottom of the deeper ocean where the depth is less than 
2\ or three miles. The oozes consist of microscopic shells of 
animals that live in the surface waters even in mid-ocean. 
When the animals die, their shells sink to the bottom, form¬ 
ing the soft and grayish deposit known as ooze. 

Deposits of the Deepest Ocean. — As the depth of the 
ocean increases, the percentage of lime in the deposits 
decreases, and at a depth of about three miles the deposit is 
chiefly red day. It seems that at these great depths the mi¬ 
nute shells and other matter of similar composition which form 
the oozes are dissolved before they reach the bottom. The 
red clay consists of the less soluble matter which settles 
from the air as volcanic ash and dust from meteors, several 
millions of which enter our atmosphere every day. Frag- 


GENERAL CHARACTERISTICS OF THE SEA 231 

ments of pumice and particles of meteoric iron occur in the 
red clay, and the insoluble parts of the bodies of animals 
living on the surface are relatively abundant. More than 
100 shark teeth and between 30 and 40 ear bones of the 
whale have been brought to the surface at a single haul of 
the dredge. Since there are but two ear bones in a whale, this 
proves that the deposit must accumulate very slowly indeed. 

Life of the Ocean. — All of the great classes of animal 
life are represented in the ocean. Several of the mam¬ 
mals, an order whose natural habitat is on land, live in 
the sea, though it is necessary for them to come to the 
surface to breathe. Among them are the whale, porpoise, 
walrus, seal, and sea lion. No birds make their permanent 
home on the sea, but many aquatic species spend much of 
their time there. Fish of great variety in size and form are 
abundant. Thousands of species of invertebrates of nearly 
every order from the microscopic protozoan to the gigantic 
squid are found in great abundance. Among these are the 
lobster, crab, shrimp, oyster, clam, starfish, and the coral. 

Various species of plants occur almost everywhere along 
the shore. A few of them, like the mangrove and certain 
grasses, are land plants which have adapted themselves to 
conditions of life on the beach; but the majority of the plants 
are unlike those on the land. Some species of seaweed 
reach great size, larger than our tallest trees, but their 
structure is unlike that of the trees, and the weight of the 
solid matter which they contain is only a small fraction of 
that of our common trees. 

Distribution of Plant and Animal Life. — The distribution 
of the life of the sea is controlled just as is that of the land, 
largely by the climatic conditions of the various parts. The 
walrus, fur seal, and narwhal are found in cold waters and 
the corals only in warm waters. The corals and certain 
allied species are also limited to the regions where the water 
is clear and normally salt; other species, like the oyster, 
prefer brackish water and do not require absolute clearness. 


232 


NEW PHYSIOGRAPHY 


The depth of water controls the distribution of life as 
effectively as any other varying condition. Light does not 
penetrate to depths much greater than 100 fathoms, and 
animals and plants requiring light must develop above this 
depth. 

The temperature of the deep ocean is near the freezing 
point; hence some forms of life are excluded. The pressure 
in the deeps is so great that other forms are excluded. And 
finally the motion of the water is so slight that fixed forms of 
life whose food must be brought to them are excluded. 

For these reasons the great depths of the sea are like the 
desert regions of the land in the comparative sparseness of 
both animal and plant life. Such animals as there are have 
strange forms; some of them have eyes, but others are 
blind. Some of the forms probably emit phosphorescent 
light which enables them to see and to be seen. There are 
no plants in the very deep sea. 

It has been maintained that the life of the sea, as a whole, 
exceeds that of the land, equal areas being compared. It is 
doubtful, however, if life is as abundant in any portion of 
the sea as it is on the more fertile portions of the land. The 
surface waters everywhere abound in life. Many species 
and many individuals of each species occur; but both the 
number of species and the number of individuals is greater 
between the 100-fathom line and the shore line than else¬ 
where. 

Ice in the Sea. — Sea water ordinarily freezes at a tem¬ 
perature between 26° and 28° F., depending upon the salt¬ 
ness of the water. In the higher latitudes ice forms along 
the shores and also on the deep sea, often to a thickness of 
eight or 10 feet. 

The ice formed in winter is usually broken in pieces in the 
summer. These floating pieces, called field or floe ice, are 
often crowded and jammed together into an ice pack, which, 
because of the lateral pressure, is raised considerably above 
the water. The sea ice may be driven upon the land by 


general characteristics of THE SEA 233 

waves and tides and become 20 feet or more thick by accu¬ 
mulations of snow. Rock fragments from overhanging cliffs 
and from the imbedding of rocks along the shore gather upon 
and in this ice of the shore, which is known as an ice foot. In 
winter the grinding of the ice foot up and down the shores 
smooths and rounds the rocks of these coasts. In the sum- 



Ice-bound shores (shaded), limits of drifting ice in northern winter (black lines), 
limits of drifting ice in northern summer (dotted lines). 

mer it breaks up and scatters the rocky material, often 
over long distances. 

Glaciers entering the sea from the land in both polar 
regions break at the shore and send off larger masses of ice 
known as icebergs. Some icebergs are a mile or more in 
length, and have been known to rise 500 feet above the water. 
As ice is nearly as heavy as water, the greater part of the 
floating iceberg is below the surface of the water. The 
relative heights above and below are on the average about 
one to eight. The chief work of an iceberg is to transport 
material in the form of bowlders and glacial pebbles, dropping 
them on the sea bottom in the warmer and more open seas. 















































234 


NEW PHYSIOGRAPHY 


QUESTIONS 

1. Where is the great water supply for watering the land? 
What other advantages does the land receive from the sea? 

2. Name the boundaries of the different oceans. Compare the 
Arctic and Antarctic Oceans in respect to area. 

3. Calculate roughly the number of cubic miles of water in 
the Atlantic Ocean. How does this compare with the volume of the 
land mass of North America? 

4. What mineral substances and gases are dissolved in sea water? 
How much common salt in 100 pounds of sea water? What causes 
the sea water to change in density in different localities? 

5. Describe the distribution of the surface temperature of the 
sea in different latitudes. Compare the temperature at the surface 
with that at the bottom of the sea, both in the higher and lower 
latitudes. Account for the striking difference in the equatorial 
regions. 

6. How are temperatures of the deep sea determined? How are 
soundings made? What is the object of dredging? 

7. Compare the ocean floor with that of the land. Account for 
differences. What is a continental shelf? About how wide are con¬ 
tinental shelves, how deep is the water upon them, and what pur¬ 
pose do they serve? What causes tend to change the area of 
continental shelves? 

8. What is the character and source of ocean bottom materials? 
How do the deposits differ in different localities? What conditions 
determine the distribution of animal and plant life in the sea ? Point 
out specific examples. 

9. Locate the two ice caps of the earth. Under what conditions 
and how is the ice formed? What is the difference between floe ice 
and icebergs? What effect does ice in the polar region have upon 
the land? 

10. Point out several ways in which the ocean is an aid to health. 


CHAPTER XV 

MOVEMENTS OF THE SEA 


The most important movements of the ocean are 
(1) waves, (2) tides, and (3) currents. 


Waves 


A gentle breeze causes ripples to form on the surface of 
water over which it blows; a strong wind changes these 
ripples into great waves. During the passage of a wave each 
particle of water affected rises and falls and moves forward 
and backward, describing a curved path in a vertical plane. 
The forward motion of the water is most rapid in the ridge 


Wind 



Fig. 103. — Diagram of Wave, Showing Movement of Water Particles 

Particle 3 is going backward, 7 forward, 5 upward, and 9 and 1 downward. 

From 1 to 5 the particles of water are going backward in the trough, from 5 to 9 for¬ 
ward on the crest, from 3 to 7 upward on the front, from 7 to 9 and from 1 to 3 down¬ 
ward on the back. 

What two motions combined has each of the following: 2, 4, 6, and 8? 

How long and how high is this wave? 

In what direction is the wave form advancing? 

If this wave should run ashore, would the water at the shore advance first or recede 
first? 


or crest of the wave, and the backward motion is most rapid 
in the furrow or trough. The forward motion is slightly in 
excess of the backward motion. Because of the excess of 
forward over backward motion of the water particles when 
the winds are long continued in the same direction, currents 
are produced which flow in the same direction in which the 

235 






236 


NEW PHYSIOGRAPHY 


wind blows. On the front of the wave the water rises, and 
on the back of the wave the water falls. As waves move, 
new water enters in front and leaves on the back of the wave. 

Height and Length of Waves. — The horizontal distance 
from the crest of one wave to the crest of the next is the 
length, and the vertical distance between the crest and the 
bottom of the trough is the height of the wave. The height 
and force of the waves depend upon the force of the wind, 
the length of time the wind continues to blow, the depth and 
breadth of the water, and the form and direction of the coast 
line. 

Groundswell. — In the open sea during a gale, waves are 
often 30 to 40 feet high and have a length of a thousand feet 
or more. High waves often pass out from an area of storm 
winds into a region of gentle winds many hundreds of miles 
away. They diminish in height, but keep their velocity and 
length. These waves that have outrun the storm which 
started them but persist after the storm are known as the 
groundswell. 

Breakers. — When a wave approaches a gently sloping 
shore, the wave length is diminished, and the wave height is 
increased. The front of the wave, because of a lack of 
water, becomes steeper than the back; and as the wave 
continues to move into water of less depth, the crest curls 
and falls forward, forming a line of breakers. At the line of 
breakers on a sandy shore a sand bar is formed. Rocks or 
bars near the surface of the water may also be located by 
breakers. Thus breakers are a warning of danger. 

Surf and Undertow. — When the waves run into shallow 
water and break near the shore, surf is formed. The water 
that is then thrown forward in the crest of the waves returns 
as a current along the bottom. This backward undercurrent 
along the bottom of a shallow sea, due to waves and surface 
currents produced by the wind, is called the undertow. When 
the waves reach the shore obliquely, a current along the 
shore is produced. 


MOVEMENTS OF THE SEA 


237 



The Work of the Waves 

Pounding of the Waves. — Waves are agents of erosion; 
that is, they break and grind the material along the shore 
and transport 4 it varying distances from the shore. 

The work of breaking and grinding is done by the fall of 
the breakers upon the shores. In summer, in the Atlantic 
the average blow of breaker is about 600 pounds on every 


Fig. 104. — A Sea Cave 

square foot of surface. In winter, the force of the breakers 
may be as high as 3000 pounds per square foot. The impact 
or pounding of the waves on the shores is made effective 
by the sand, the pebbles, and such rock fragments as the 
waves are able to move. Driven by the force of the waves, 
they serve as tools for cutting and grinding and become 
rounded by acting upon each other. Weak rocks exposed 
along the shores are broken down and removed. The more 




238 


NEW PHYSIOGRAPHY 



resistant rocks are loosened by undercutting, and because 
of the joints and seams in the rocks fall as angular blocks. 
These angular blocks in course of time become reduced in 
size and rounded. Large masses of rock, too large at first to 
be moved by the waves, are reduced by the pounding of 
smaller fragments which the water drives against them until 
they, too, are shattered. Then the waves use these new 
fragments, in their turn, as weapons of attack. Thus huge 
masses of rock are reduced in turn to cobbles, pebbles, and 


Fig. 105. — Action of Waves, Showing Tendency to Follow Joints in the Rocks 

sand, and finally to the finest mud particles, which may be 
carried away by the undertow. 

Sea Cliffs and Sea Caves. — The cutting of the waves at 
the water level may be compared to a horizontal saw. As 
the waves cut into the shore, the unsupported material 
often falls, leaving a steep face known as a sea cliff. If the 
sea cliff is a wall of rock, and the waves continue undercutting 
at the base, a sea cave may be formed. 

Sea Arches and Chimney Rocks. — If the wearing away 
of the roof continues, the remaining portion may form an 
arch or bridge. Sometimes the waves remove block after 




MOVEMENTS OF THE SEA 


239 


block of rock along certain joints, so that a column or pillar 
of rock may be isolated from the shore. These are then 
known as chimney or pulpit rocks. The “ Old Man of Hoy ” 
on the coast of the Orkney Islands is an example. 

Small irregularities in the shore line develop because of 
differences in the resistance of the rocks and in their exposure 
to the attack of the waves, but as a rule the action of waves 
and shore current tends to make the shore line more regular; 
the projecting headlands are worn away and bay heads are 
filled. 

In certain places waves wear away the land and deposit 
the material in the sea at a lower level. The rock fragments, 
pebbles, and sand formed at the shore are ground finer and 
carried away by the combined action of waves, undertow, 
and along-shore currents. 

Deposition by Waves, Undertow and Shore Currents. — 

In other localities material is brought in from the sea by 
the waves and deposited on the shore within the zone of 
wave action and forms the beach. When the material carried 
out by the undertow meets that brought in by the waves, 
an accumulation begins at the place of meeting. A low 
ridge called a barrier is formed, and its position is shown by 
the line of breakers. Such barriers are often built up to 
and above the surface of the water, making a sand reef. 

The free end of a beach or a barrier is called a spit. The 
deposits along the shore depend largely upon the shore 
currents. The growth westward of Rockaway Beach, on 
the southern shore of Long Island, is due partly to along¬ 
shore currents in that direction. 

The constant growth of deposits brought down from the 
shore tends to fill up bay entrances and, consequently, to 
interfere with navigation. At the entrance to New York 
Harbor, for example, dredging is necessary at all times to 
deepen the channels through which the largest boats pass. 
A fleet of dredges is constantly at work not only in the bay 
but also in that arm of the sea called the East River. 


240 


NEW PHYSIOGRAPHY 


Tides 

Tides Defined. — Along the shores of the ocean and its 
gulfs and bays the water rises slowly for about 6 hours and 
13 minutes, and then falls slowly for about the same time, 
making on an average 12 hours and 26 minutes from high 
water to next high water, or from low water to next low water. 
This 'periodic rise and fall of the level of the sea twice in every 
2^ hours and 52 minutes constitutes the tides. 

This makes the hour of high water at any particular place 
vary from day to day. If it is high water at the ocean shore 
this afternoon at 4 o’clock, the next high water will occur 
again at 4:26 to-morrow morning, and high water again at 
4:52 to-morrow afternoon, and so on. 

Variation in Tidal Range. — The amount of rise and fall 
is greater along most continental coasts than in mid-ocean, 
and greatest in bays with broad openings to the sea and 
narrow toward their heads. The tidal range at Key West, 
Florida, is usually not more than two feet, while in the Bay 
of Fundy it is often more than 50 feet. 

The amount of the rise and fall of the sea at any particular 
place also varies. The tidal range may increase from day to 
day for about a week and then decrease for the same period, 
making a maximum and minimum range twice a month. 
At Governor’s Island in New York Harbor the tidal range 
may be as small as 3.4 feet and as great as 5.3 feet during a 
single week. 

Flood and Ebb Tides. — The change of level of the sea 
is accompanied by tidal currents called the running of the 
tides. When the tide is running from the open ocean into 
bays, it is flood or incoming tide; and when the tide runs to 
the open ocean again, it is the ebb or outgoing tide. During 
the few minutes when the flood tide changes to ebb tide 
or ebb to flood, slack water occurs. 

Tidal Races. — When the tidal currents pass through a 
strait, such as a narrow inlet into a bay or between an island 


MOVEMENTS OF THE SEA 


241 


and the mainland, the currents often run many miles an 
hour. Such currents are called tidal races, and are often so 
strong as to interfere with navigation. The tidal currents 
“ race ” through Hell Gate, the narrow passage from the 
East River into Long Island Sound, at the rate of five or 
six miles an hour. 

Tides in Rivers. — The tidal wave often runs up rivers 
to a point many feet above sea level. The tide runs 150 miles 
up the Hudson River to Troy, five feet above sea level, 
where the tidal range is more than two feet. The tide is 
felt 70 miles up the St. John River in New Brunswick, 
where the elevation is 14 feet above sea level; and at Mon¬ 
treal, 280 miles up the St. Lawrence River. 

The action of tidal currents in narrow rivers is very differ¬ 
ent from the action of tidal currents on open seacoasts. In 
rivers, when the water stands above the average level, the 
tidal current flows upstream along with the tidal wave; 
and when the water stands below the average level, the 
tidal current flows downstream, opposite to the direction of 
the tidal wave. Since the rate of flow depends upon the 
difference in level, the flow is most rapid at high and low 
water instead of being slack water at these times, as on 
open coasts. Hence the tidal current flows upstream for 
some time after high water has passed and the water level is 
falling; and the tidal current flows downstream for some 
time after low water is reached and the water level is rising. 
In broad, deep mouths of rivers, slack water does not occur 
at high and low water as .on open coasts, nor at average 
level as in narrow shallow rivers, but at some intermediate 
level. 

Tidal Bore. — In the estuaries of many rivers, broad flats 
of mud or sand are nearly exposed at low water. The tidal 
wave when entering these rivers often rises so rapidly that it 
assumes the form of a wall of water. Such a wave is called a 
bore. Tidal bores occur in some of the rivers of China, where 
in one case the bore travels up the river at every high tide, 


242 


NEW PHYSIOGRAPHY 


often reaching a height of twelve feet. After the bore has 
passed, an after-rush often carries the water up several feet 
higher. 

Bores have been observed on the Severn in England, on the 
Seine in France, on the Amazon in South America, and on a 
few other rivers of the world. 

Causes of the Tides. — Since Newton announced the law 
of universal gravitation, it has been generally recognized 
that the tides result from the attraction of the sun and moon. 
The tide-producing forces of sun and moon can be computed 
with reasonable certainty; but because of the modified 
effects due to local conditions, an agreement between theo¬ 
retical and the actually observed tides is not easily secured. 
Although the moon’s mass is only a small fraction of the sun’s 
mass, the moon’s nearness to the earth makes it, rather than 
the sun, the principal cause of the tides. 

First Law of Motion. — A body in motion will move in a 
straight line unless deflected from its straight path by some 
external force. This law of motion may be illustrated by 
whirling a stone around the hand by means of a string. The 
natural tendency of the stone, at each instant, is to move in 
a straight path. It is deflected and moves in a curved path 
because of a pull or force, called centripetal force , exerted 
by the string acting inward upon the stone. The stone re¬ 
sists being pulled inward and so tends to move outward 
and exerts a pull or force upon the hand called centrifugal 
force. The string, being under tension when the stone is 
whirled, is subject to equal and* opposite forces, one acting 
toward (centripetal) and the other away from (centrifugal) 
the center of revolution. 

Balance between Centripetal and Centrifugal Forces. — The revo¬ 
lution of the moon about the earth is illustrated by this simple 
experiment. The invisible force called gravitation which acts 
between the moon and the earth replaces the centripetal force 
exerted by the string that holds the stone to the hand. The moon 
whirls about the earth with sufficient velocity and at such a distance 


MOVEMENTS OF THE SEA 


243 


that her resistance to curved motion, or centrifugal force, just equals 
and balances the attraction between the earth and moon. 

Center of Gravity. The moon does not revolve about the center 
of the earth, but about a point 3000 miles from the center or 1000 
miles below the surface. This is 
because the earth is 80 times as 
heavy as the moon, and the centers 
of the two bodies are 240,000 miles 
apart. 

This may be easily illustrated by 
balancing two balls, one 80 times 
the weight of the other, connected 
by a slender rod. The place where 
they balance, called the common center 
of gravity , will be one-eightieth of 
the distance from the center of the 
larger ball to the center of the smaller. 

Revolution about Common Center of Gravity. —- The common cen¬ 
ter of gravity of the earth and moon is at C. The big and little balls 
correspond to the earth and the moon, and the stress in the rod 
represents the attraction that holds the earth and moon together. 
Both the earth and the moon revolve about this common center of 



Fig. 106. — Resultant Fokces 



gravity, C, in about 28 days, and in so doing the earth’s center 
describes a circle with a radius of 3000 miles. 

The daily rotation of the earth, which is not now being considered, 
must not be confused with the revolution of the earth, without 
angular turning, about a point 1000 miles below the earth’s surface. 
Only the earth-moon revolution about C without rotation of either 
body is here considered. When a body revolves about another 
without rotation, a given side always faces the same direction in space. 



244 


NEW PHYSIOGRAPHY 


Revolution without Rotation. — It may be stated as a general prop¬ 
osition that whenever an object revolves without rotation, every 
particle of the object describes a path the size and shape of that 
described by a particle at the center of the object. The motion of 
the different particles of a connecting rod attached to the driving 
wheels of a locomotive illustrates this action. 

All parts of the earth then must be subject to equal and parallel 
centrifugal forces, because of the monthly revolution of the earth 
and moon about their common center of gravity. These forces act 
in a direction away from the moon. The total of centrifugal forces 
acting on the earth is just balanced by the total centripetal force 
due to the moon’s attraction, although it is evident that the two 

B 


• Producing Force 

D 

Fig. 108. — Resultant Forces 

opposite forces acting on any single particle are only equal at the 
center of the earth. 

Unequal Attraction of the Moon in Different Parts of the Earth. — 
The moon’s attraction for the earth is always toward the moon, but 
is not equally distributed, for the attraction on the side of the earth 
nearest the moon is stronger than at the center, and on the side of 
the earth farthest from the moon weaker than at the center. 

Resultant of Two Opposite Forces. — In the figure, ABCD, repre¬ 
senting the equator of the earth, A is a particle farthest from the 
moon; C a particle nearest to the moon, and E a particle at the cen¬ 
ter of the earth. The arrows of equal length, extending to the 
left away from the moon, represent the equal centrifugal forces; 
and the arrows of unequal lengths, extending to the right toward 
the moon, represent the unequal value of the moon’s attraction at 
these points. 

When two forces act in opposite directions at the same point, the 






MOVEMENTS OF THE SEA 


245 


effectiveness or resultant of the two forces is found in a force equal 
to the difference between the two and acting in the direction of the 
greater force. 

At C the moon’s attraction is greater than the centrifugal force 
at that point, so that the tide-producing force, which is the difference 
or resultant between these forces, acts toward the moon and causes 
the water on the side of the earth toward the moon to bulge out 
toward the moon. 

At A the moon’s attraction is less than the centrifugal force; and 
the' tide-producing force consequently acts away from the moon, 
causing the water on the opposite side of the earth to bulge out away 
from the moon. 

At E the moon’s attraction and the centrifugal force are equal 
and opposite. If they were not, the earth and moon would either 
approach or recede from each other. 

These two bulges of the ocean are the two high tides, and mid¬ 
way between them is the low-tide zone. 

The magnitude and direction of the resultant or tide-producing 
forces acting at different points on the earth’s equator are shown 
in Figure 108. 

Effect of Rotation of the Earth. — The daily rotation of 
the earth from west to east constantly carries the high and 
low tide westward around the earth and brings places alter¬ 
nately to high tide and low tide positions. 

The tidal movements are interfered with by the continents, 
which tend to stop or change the direction of the tidal wave. 
The tidal wave travels faster in the deep ocean than in the 
shallow water near the continents. The tidal waves are also 
interfered with by the strong winds and changes of atmos¬ 
pheric pressure. Their advance in different parts of the 
ocean becomes so irregular that they often interfere with 
one another. This explains in some measure why the actual 
local tides in so many places fail to agree with the general 
theory. 

The Establishment of the Port. — The rotation of the 
earth tends to carry the tidal waves forward in the direction 
of the rotation. The moon tends to hold the tidal waves 


246 


NEW PHYSIOGRAPHY 


back. The result is that the tides are said to lag. The 
interval of time between the passage of the moon across the 
meridian and the next high tide, mariners call “ the estab¬ 
lishment of the port.” The establishments of different ports 
have various values. The port of New York has a value of 
eight hours and thirteen minutes. 

Cause of Solar Tides. — The explanation of solar tides is 
analogous to that of lunar tides. Since the cause of lunar 
tides is the difference between the moon’s attraction and 
centrifugal force in different parts of the earth, in like manner 
solar tides are due to the difference between the sun’s attrac¬ 
tion and centrifugal force in different parts of the earth, 
caused by the earth’s moving about the common center of 
gravity of the earth and the sun. 

Effect of Solar upon Lunar Tides. — The intensity of the 
tide-producing force due to the sun is about half of that 
due to the moon. Since the lunar tides are stronger than the 
solar tides, the solar tides may be said to modify them; 
that is, to strengthen the tides when sun and moon act 
together and to weaken them when they oppose each 
other. 

Twice a month, at times of new and full moon, the lunar 
and solar tides fall together, producing a higher tide than 
usual. This condition of greatest range is called spring tide. 
At first and last quarters of the moon the solar high tide 
falls at lunar low tide, and solar low tide falls at lunar high 
tide. The effect of this is to lessen the tidal range; that is, 
the high tides are not so high and the low tides are not so 
low as usual. This condition of least range is called neap tide. 

The relative ranges of spring and neap tides may be shown 
graphically by the construction of tide curves for any 
station. The data for these tide curves may be found in 
tide tables published by the Government. 

The tides in any latitude vary with the changing angular 
distance of the moon and sun north or south of the equator, 
as well as with their changing distances from the earth. 


. 109. — Diagram of a Tidal Curve 


MOVEMENTS OF THE SEA 


247 











































































































































































































































































































248 


NEW PHYSIOGRAPHY 


Inequality of Tides. — The two successive high tides of a 
given place are usually of unequal height. They are of 
equal height only when the moon is over the equator, and 
as this happens on only two days of the month two weeks 
apart, the two successive high tides are usually unequal. 
The maximum inequality of successive high tides occurs 
when the moon is farthest north or south of the equator. 
This variation at some places amounts to several feet. 

Maximum Yearly Tide. — The conditions that favor the 
greatest tidal range in any particular harbor are: (1) new 
or full moon, (2) moon and sun nearest to the earth, (3) 
moon and sun’s zenith distances approximating the latitude 
of the place affected, (4) wind direction favorable to direc¬ 
tion of tidal movement. 

Effect of Tides. — The erosion caused by tidal currents is 
known as tided scour. The tidal scour of the flow and ebb 
of the tide maintains inlets in barrier reefs along many 
shores. An example of this may be seen in the sand reefs 
along the shore of New Jersey. Tidal scour also often main¬ 
tains deep waterways in some bays to the advantage of 
navigation; whereas at the entrance to other bays the tidal 
currents tend to fill, making the water shallow, and because 
of shifting of deposits are dangerous to navigation. Strong 
tides hinder the formation of beaches across the entrance 
of some bays. 

The tidal currents cause a circulation of water in bays and 
harbors which prevents stagnation and helps to remove the 
sewage that is drained into them by neighboring cities. 
This circulation of water aids or hinders boats, according 
to their direction, and sometimes drifts vessels out of their 
course and subjects them to danger of rocks and shoals, 
especially in times of dense fogs. 

Tidal currents transport material along shore from more 
exposed positions, such as headlands, to the less exposed 
position at the heads of bays. This filling of bay heads 
tends to straighten the shore line. 


MOVEMENTS OF THE SEA 


249 


Currents 

Every continent is washed by ocean currents, and every 
ocean has its distinct circulation. Currents from equatorial 
regions carry warm water into polar regions, and other 
currents carry the cold polar waters into lower latitudes. 

While each ocean has its separate circulation, yet the 
separate schemes of circulation fit into the general scheme 
as cogwheels in a vast machine. 

The Pacific Ocean, which for most purposes is considered 
as one ocean, is by reason of its circulation divided into two 
distinct parts, the North Pacific and the South Pacific. 
The Atlantic and Indian Oceans lying, like the Pacific, on 
both sides of the equator, are also divided into northern and 
southern oceans by reason of their distinct circulation. 

Systematic Movement. — Ocean currents, like air cur¬ 
rents, obey Ferrel’s Law, in that they turn to the right of a 
straight course in the northern hemisphere, and to the left 
in the southern. This results in a distinct eastward drift 
about the margin of the south polar ocean, and a less distinct 
eastward movement about the Arctic Ocean. In other oceans 
the northern divisions have a clockwise circulation, whereas 
the southern divisions have their circulation counterclock¬ 
wise. 

The movement of the waters in all oceans is chiefly about 
the margins, leaving the great central areas undisturbed. In 
these areas of quiet water seaweed and other floating matter 
accumulates, thus producing what are known as Sargasso 
Seas. These seas are avoided by masters of sailing vessels, 
who find it difficult to get out of these drift-covered waters 
when driven into them by storms. Columbus thought, when 
he came to the Sargasso Sea in the Atlantic, that he had come 
upon land. 

Cause of Currents. — All winds, however fitful, brush the 
surface water along with them. If they constantly vary in 
direction, no systematic nor continuous currents can result. 


250 


NEW PHYSIOGRAPHY 


When the same direction is held for several days, a distinct 
drift with the wind is observed. 

Continued east winds over Lake Erie have at times so 
heaped up the water toward the west end of the lake that 
Niagara Falls has practically run dry; whereas a continued 
west wind brings an unusual volume of water over the falls. 
We are told, too, that strong east winds sometimes drive 
the waters back from one of the arms of the Red Sea and 
make it possible to cross this basin “ dry-shod.” 

Other minor causes may operate to produce locally current¬ 
like movements by causing differences of level in the ocean 
surface. The excessive rainfall in the doldrum belt combined 
with excessive evaporation in the trade-wind belts tends to 
cause northward and southward movements of the surface 
waters; and great storms, like the Galveston storm, pile 
the water up against the land to be returned as local currents. 
Differences of temperature produce vertical currents when 
the surface water is colder than the water below, but hori¬ 
zontal variations in temperature can scarcely cause percep¬ 
tible motion. 

Origin of Ocean Currents in the Trades. — Since con¬ 
tinued wind from a given direction sets the water drifting 
with it, the trade winds, blowing as they do always from the 
same direction, would seem the logical cause of those world¬ 
wide movements found in all oceans known as ocean currents. 

Near the eastern sides of the oceans, where the trade 
winds are not well established, the currents are weak and 
somewhat irregular; but farther west, away from the dis¬ 
turbing influence of the continents, the currents both north 
and south of the equator are pronounced and continuous. 

The current under the northeast trades is known as the 
north equatorial current, and that under the southeast trades 
the south equatorial current. These are found in the three 
oceans that lie athwart the equator; and between them is 
found an eastward- moving current known as the equatorial 
counter current , to be explained later. The north and south 






















• 1 


* 

1 -V * ht ,: *i»W 















* 




















































































































































































MOVEMENTS OF THE SEA 


251 


equatorial currents may be considered the birthplaces of the 
world-wide system of ocean circulation. 

Poleward Currents. — The equatorial currents are barred 
in their westward movement by islands and continents 
across their paths. They are thus forced to turn poleward 
along the western shores of the oceans. Whether they turn 
northward or southward is determined by the outline of the 
coast. 

While the currents are moving along and near the equator, 
the earth’s rotation has but slight deflecting influence; and 
it is probable that, if not interrupted by land barriers, the 
equatorial currents would continue their westward course 
around the earth. 

As soon, however, as they begin to flow into other lati¬ 
tudes, the rotation of the earth is effective in turning them 
from a straight course, to the right in the northern hemi¬ 
sphere and the left in the southern. 

These poleward currents are warm currents and carry the 
warm water from the equatorial regions into colder latitudes. 
At the same time they spread out, lose their velocity, and 
are then known as drifts, which move to the margins of the 
polar oceans, then eastward to the eastern shore of the 
ocean in which they have their origin. 

Equatorward Currents. — By continued deflection, these 
eastward-moving currents, now cooled from loitering in 
high latitudes, are in part turned toward the equator along 
the western coasts of the continents. Returning thus to 
the trade-wind belts, in which they assume their westward 
direction, the circulation about the nonpolar oceans is com¬ 
plete. Other branches of these eastward-moving currents 
are, by the configuration of the land or sea bottom, made to 
take other courses. 

The equatorward currents are cold or cool currents and 
bring lower temperatures toward or even to the equator, 
causing the eastern sides of all oceans in the lower latitudes 
to be cooler than the western sides. 


252 


NEW PHYSIOGRAPHY 


Circumpolar Currents. — Under the winds of the circum¬ 
polar whirl the waters of the polar oceans move with them, 
counterclockwise in the Arctic Ocean and clockwise in the 
Antarctic. The movement of the currents about the 
Arctic Ocean is less well developed and not so strong as 
that about the Antarctic, because of the numerous islands 
in the north that interrupt. Branches from the circumpolar 
movement in the north are sent off southward into the 
Pacific and the Atlantic. These cold currents, deflected to 
the right, follow closely the eastern coasts of Asia and North 



Fig. 111. — Currents of the Indian Fig. 112. — Currents of the Indian 
Ocean in January Ocean in July 


America, until they sink beneath the warm currents between 
the parallels of 40° and 50° N. Because of the unobstructed 
course of the Antarctic Drift, it flows eastward along the 
border of the Antarctic continent with greater velocity than 
the Arctic Drift has. The “ brave north westerlies ” that 
blow in this high southern latitude are likewise responsible 
for the greater velocity of the currents there. 

Creep. — Their further journeying toward the equator 
is known as creep. In this way the cold polar waters are 
carried even to the equator, and the low temperatures of 
deep equatorial seas are accounted for. We cannot observe 
the creep; but as more surface water is carried into polar 























MOVEMENTS OF THE SEA 253 

regions than returns as surface currents, the excess must be 
equalized by under-surface return currents. 

Monsoon Currents. — If any doubt existed as to the suffi¬ 
ciency of the winds to produce ocean currents, that doubt 
would be removed by a study of those currents which change 
their direction with the change of direction of the monsoons. 

While there are monsoons at the horse latitudes, the winds 
there are of neither sufficient strength nor constancy to be 
effective in producing ocean currents. It is in the monsoon 
belt over which the heat equator migrates that we find con¬ 
ditions favorable for the production of ocean currents. 

Currents of the Northern Indian Ocean. — About the 
northern Indian Ocean, when the southwest monsoon 
blows, the water is set drifting in a clockwise direction. 
As these winds weaken, this drift slackens; and soon after 
the northeast monsoon begins, the direction of the drift is 
reversed. It continues as a counterclockwise circulation 
while the northeast monsoon continues, changing again to 
the clockwise direction with the return of the southwest 
monsoon. These changes of direction of the ocean currents 
can be accounted for only by the reversal of the winds. 

Equatorial Countercurrents. — In the Pacific Ocean, 
where the heat equator lies prevailingly north of the terres¬ 
trial equator, the southeast trades, changed to southwest 
winds north of the equator, set up an ocean drift to eastward. 

This is the equatorial countercurrent. It is fairly distinct 
throughout the year, though better developed during the 
northern summer. Its explanation is the same as that of the 
clockwise movement about the northern Indian Ocean during 
the southwest monsoon. 

Because of the narrowness of the Atlantic Ocean at the 
equator, the countercurrent is not so well developed as in 
the Pacific. 

Currents and Navigation.—Sailing vessels lay their courses 
to suit the winds and ocean currents, and even steamships do 
not scorn to take advantage of the great ocean circulation. 


254 


NEW PHYSIOGRAPHY 


Sailing vessels from New York to English ports take 
advantage of the northeast Atlantic Drift; on their return 
they use the trades. Those bound from New York to Rio 
Janeiro must lay their courses far to eastward of the eastern 
cape of South America, lest the equatorial currents carry 
them northward again while in the doldrums, where winds 
are apt to fail. 

Ships sailing from Atlantic ports for Australia sail east¬ 
ward around the Cape of Good Hope, to take advantage of 
the Antarctic Drift, while those returning also sail eastward 
past Cape Horn, to have the advantage of the same drift. 

Vessels bound from Honolulu to San Francisco sail north¬ 
ward beyond the trades and equatorial current, then east, 
returning, they take a more southerly route. 

Currents and Life. — The distribution of many marine 
forms is determined by the temperature of the water, which 
in turn is in part determined by ocean currents. Corals 
serve well to illustrate. The waters of the Galapagos Islands, 
west of South America, are too cold for corals, although these 
islands lie under the equator. The Peruvian Current, bring¬ 
ing water from the Antarctic regions, makes these waters 
cold. Contrasted with these islands are the Bermudas in 
the Atlantic in latitude about 35° N., which are chiefly 
coral rock and are bordered by reefs of living coral. The 
warm waters are brought to these islands by the Gulf Stream. 

The seeds of many plants are distributed by means of 
ocean currents, and insects and the smaller animals are 
carried upon drifting materials in these currents. 

Currents and Climate. — The direct climatic influence of 
ocean currents is confined to the ocean and immediately 
bordering lands. Indirectly their influence may be felt 
hundreds of miles inland. This is markedly true of lands 
lying to leeward of currents that are abnormally cold 
or warm. 

The North Atlantic Drift. — The most pronounced and 
far-reaching of all ocean currents in its climatic influence is 


MOVEMENTS OF THE SEA 


255 


perhaps the Gulf Stream. The winds from over this broad 
sheet of warm water not only bring abundant rainfall to the 
British Isles and Norway, but so temper the cold of these 
high latitudes as to make them comparable in temperature 
to our own eastern coasts, 20° farther south. 

The North Pacific Drift. — The continuation of the Japan 
Current tempers the climate of Alaska and British Columbia 
in like fashion. 

These great drifts, in both oceans, continue or send 
branches southward along the western coasts of the conti¬ 
nent; and when they reach the latitude of northern Mexico 
and Africa, their effect is to temper the heat of these coasts. 

The cold currents that follow closely the eastern coasts of 
North America and Asia, being to leeward of those conti¬ 
nents, do not affect the climate so far inland. However, 
the bleakness of Labrador and Kamchatka is in some degree 
traceable to these currents. 

In the southern hemisphere the western coasts are cooled 
and the eastern coasts warmed by the ocean currents, but 
their influence is less pronounced than in the northern 
hemisphere. 

Currents and Harbors. — The harbor of Hammerfest, 
at the north of Norway and well within the Arctic Circle, 
is about as free from ice as that of Boston, 30° farther south. 
In the one case we see the effect of the warm North Atlantic 
Drift; in the other, of the cold Labrador Current. 

In the Pacific Ocean the barrier of the Aleutian Islands, to¬ 
gether with the narrowness of Bering Strait, prevents the 
North Pacific Drift from entering the Arctic Ocean. As a 
result, the bays on the north coast of Alaska, in the same 
latitude as Hammerfest, are practically closed by ice through¬ 
out the year. 

The Russian-Japanese War had for one of its objects the 
securing for Russia of the open harbor of Port Arthur. The 
harbor of Vladivostock, Russia’s chief port on the Pacific, 
in about the latitude of New York, is for a long time every 


256 


NEW PHYSIOGRAPHY 


year closed by ice, owing to the cold current coming down 
through Bering Strait. 

The Gulf Stream. — This greatest and most important 
of all ocean currents derives its name from the Gulf of 
Mexico, from which it issues. It is in fact a continuation of 
the combined equatorial currents. 

The North Equatorial Current in the Atlantic is turned by 
the land masses in its path wholly into the northern division 
of this ocean. Much of its waters pass among the islands of 
the West Indian group, while the remainder passes to the 
eastward. 

The eastern cape of South America is so situated that it 
divides the South Equatorial Current in two, part of it 
turning southwest along the coast of Brazil as the Brazilian 
Current, while the other part enters the Gulf of Mexico be¬ 
tween the West Indies and the mainland of South America. 
This water issues through the Strait of Florida as the 
Gulf Stream. It is truly a stream , flowing between banks of 
water. At that point it is deep and narrow, scouring the bot¬ 
tom of the strait, and, it flows with a velocity greater than 
that of the lower Mississippi River. 

Joined by the waters that come through the West Indian 
group of islands and that which passes outside, the Gulf 
Stream is greatly increased in volume. It passes parallel 
to and near enough to the Carolina coasts to send off return 
eddies, which build the Carolina capes. Spreading and 
decreasing in velocity, the Gulf Stream becomes the North 
Atlantic Drift. 

The frequent and dense fogs off Newfoundland are pro¬ 
duced by warm winds from the North Atlantic Drift, blowing 
over the cold Labrador Current. The line of meeting of the 
cold and warm water is known as the cold wall. 

QUESTIONS 

1. Tides resemble waves in many respects. High and low tides 
correspond to what parts of the wind wave? Tidal currents corre- 


MOVEMENTS OF THE SEA 


257 


spond to what phenomenon of the wind wave? The change in tidal 
range, the velocity and form of the tidal wave as it advances in 
shallow water on the continental shelf and into bays, may be com¬ 
pared to what changes in the wind wave as it moves toward the 
shore? Compare the height and length of wind waves with that of 
tidal waves. 

2. Is seasickness more likely to occur on large or small boats? 
Why? What is the difference between surf and a breaker? What 
work is done by breakers and the undertow? Why are breakers 
a warning of danger? 

3. Explain how the waves act as a horizontal saw cutting into 
the land. What are some of the shore features resulting from wave 
action? What effect has these features upon the value of harbors 
and shore property? 

4. How would a thoughtful person living at the shore for any 
length of time naturally connect the cause of the rise and fall of the 
sea with the moon? 

5. Explain how navigation is affected by (a) tidal range, ( b ) flood 
tide, (c) ebb tide, ( d ) tidal races, (e) tidal bores. How do you think 
the state of the tide affects fishing? 

6. How can one moon cause two daily tides, or, in other words, 
what is the cause of a high tide on the side of the earth opposite to 
that of the moon? 

7. Which has a lower low water, a spring or a neap tide? Ex¬ 
plain. How often does the moon cross the equator? What effect 
has this on the height of the two daily tides? 

8. What effect has tidal scour upon waterways, inlets, and tidal 
streams? What is the general effect of tides upon the water and 
shores in and about bays and harbors? 

9. What are ocean currents? How fast do they flow? How deep 
are they? Describe a particular current in detail. 

10. What is meant by the cogwheel scheme of circulation? What 
is a Sargasso Sea? 

11. What is the general cause of ocean currents? Point out 
definite evidence. Name and locate several ocean currents. What 
is a “creep”? 

12. What is the effect of ocean currents upon climate? Point 
out specific examples. What is the effect of ocean currents upon 
navigation? Point out specific examples. 





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t 





















PART IV 


THE LAND 


















































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CHAPTER XVI 


THE MANTLE ROCK 
Structure of the Solid Earth 

Every one is familiar with the fact that solid rock appears 
on the surface of the land in but few places and that this 
surface nearly everywhere consists of loose or unconsolidated 
earthy matter. This is the mantle rock. In some places it 
reaches a thickness of several hundred feet; but as a rule, 
the full thickness is revealed in stream valleys, and one 
can find such sections as that shown in Figure 113 in nearly 
all ravines. 

The solid rock which underlies the mantle rock is called 
the bedrock. In the ordinary sense the term rock does not 
include loose, fragmental deposits, but natural formations 
of the same origin show all degrees of consolidation from 
that of sand to the hardest sandstone. We therefore define 
rock as a natural deposit of earthy matter , whether consolidated 
or not. 

When we speak of rock as a material, we call it stone in 
contradistinction to wood, metal, etc. 

Economic Importance of the Mantle Rock. — The fact 
that the mantle rock consists of loose materials facilitates 
many of the operations that men undertake, and materially 
lowers their cost. All grading is much simpler and cheaper 
than on solid rock; excavations for buildings cost but a 
fraction of what they would cost in bedrock, and agriculture 
is possible only in mantle rock. 

Because of its porosity, most of the mantle rock permits 
water to pass through it freely. A portion of the rainfall, 
therefore, sinks into the ground and slowly descends until it 
comes to some impervious layer such as clay. Here its 

261 


262 


NEW PHYSIOGRAPHY 


downward motion must cease, and there is usually a slow 
motion sidewise toward regions receiving less rainfall or 
toward valleys in which the impervious layer comes to the 
surface. 

1. The permeability of the mantle rock thus makes it an impor¬ 
tant medium for the distribution of water. 

2. It also makes it a great reservoir which temporarily stores a 
large portion of the rainfall, thus tending to prevent floods which 
would otherwise occur after every heavy rainstorm. The quantity 



Fig. 113 .— Natural Section Showing Mantle Rock and Bedrock 
Lockport, New York. Photo by Geological Survey of New York. 

thus conserved is much greater than that conserved by the forests, 
important as is this latter amount. 

3. The water in this reservoir supplies wells and springs, keeps 
plants alive in dry weather, and supplies streams with water during 
dry seasons, making the volumes less variable. 

4. The mantle rock is a natural filter . Rain washes the air, beating 
down dust particles and removing disease germs. On the surface 
of the earth it becomes muddy and contaminated in many ways, 
making the surface water unsafe for household use. The water of 
the wells and springs is clear because the mantle rock has filtered it; 
and if wells are not too shallow, the water is generally pure and safe 
to use. 

5. The mantle rock is a great storehouse of plant foods such as 
potash, phosphorus, and mineral salts. 





THE MANTLE ROCK 


263 



Origin of the Mantle Rock. — The mantle rock consists of 
fragments of bedrock, in various stages of disintegration and 
decay, that have been loosened and changed through the 
action of a number of natural agents which accomplish the 
result in different ways. 


The quiet action of the atmosphere, with its moisture and its 
changes in temperature, is of great importance in disinte¬ 
grating the solid rock. In this manner much of the mantle 
rock has been formed. 

Glaciers and running water wear away the surface of the 
rock over which they move and add the loosened particles 
to the mantle rock. 


Fig. 114 . — Weatheeed Granite 
Photo by United States Geological Survey. 










264 


NEW PHYSIOGRAPHY 


An appreciable addition to the mantle rock results from the 
action of wind-blown sand and the waves on solid rock. Fig¬ 
ure 118 shows rock that has been much worn by wind-blown 
sand and Figure 104 rock worn by wave action. 

Weathering.—Every boy has learned that the stones found 
in the fields differ greatly in hardness and strength. Some¬ 
times one finds a stone which will crumble in one s hands or 
scale off on the outside but which is well preserved and hard 
in the center. Such specimens illustrate weathering. 

The difference in the appearance and the solidity of freshly 
quarried rock, and that of the same rock which has been 
exposed long to the action of the elements, is due to weath¬ 
ering. The stones of many buildings less than a quarter of 
a century old show the effect of weathering, and some of 
the stones that are used extensively for building in the 
United States weather to such an extent in a few years that 
it is necessary to protect them in some manner to prevent 
their entire destruction. 

Weathering is the term applied to the various natural proc¬ 
esses of softening and disintegrating the surface layers of rock 
exposed to the atmosphere. It includes the processes of solu¬ 
tion, abrasion, and corrosion. Corrosion is the term applied 
to the slow processes of weathering due to chemical action. 

Chemical Weathering. — Certain agents of weathering 
attack rock in practically the same way that articles made 
of iron are attacked when they rust. These agents produce 
chemical changes in the rock, and the products of their 
action are new substances unlike the original, just as iron 
rust is unlike the iron from which it was formed. The most 
important chemical agents concerned in weathering are 
oxygen, carbon dioxide, and water. 

Oxygen. — This is the most active of the elements in the 
air. In the presence of moisture it not only combines with 
iron and a number of other metals, but it also attacks many 
compounds found in the rocks, uniting with them and form¬ 
ing new compounds. Rock ledges sometimes show the effects 


THE MANTLE ROCK 


265 



Fig. 115 . — Hoodoo Basin, Ahoaraka Range, Yellowstone Park 

Showing fantastic forms carved from igneous rock by rain and weathering. 
Photo by United States Geological Survey. 


of the action of oxygen several inches from the exposed sur¬ 
face. The oxidized portion is easily distinguished by a differ¬ 
ence in color, and by the ease with which it is disintegrated. 












266 NEW PHYSIOGRAPHY 

Carbon Dioxide. — This is another constituent of the air 
which corrodes rock. It is most active when dissolved in 
water. The igneous rocks are largely composed of complex 
minerals and are decomposed by water containing carbon 
dioxide. When the constituent minerals contain calcium, 
one of the products of this action is calcium carbonate. 
Being soluble, the calcium carbonate thus formed is carried 


Fig. 116 . — Exfoliate Weathering of Granite 
Photo by United States Geological Survey. 

to the sea by streams, where much of it reappears in solid 
form as limestone. 

Water. — Water often combines with some of the constitu¬ 
ents of rocks, with an increase in volume which causes the 
remainder of the rock to crumble. Certain micas illustrate 
this action, and this probably accounts for the rapid weather¬ 
ing of micaceous sandstones. 

Other Chemicals. — Nitric acid formed in the air by light¬ 
ning, certain sulphurous gases erupted by volcanoes, and 









THE MANTLE ROCK 


267 


acids formed by decaying vegetation also produce chemical 
changes in rocks which result in their disintegration. 

Mechanical Weathering. — Certain agents abrade rocks 
in the same way that a file wears away iron. This is a 
mechanical process, and the products remain the same ma¬ 
terial as the original substance, just as iron filings are the 
same material as the piece of iron from which they were 
separated. Other agents disintegrate rocks by blows like 
those of particles of flying sand. All of the agents which 
disintegrate without changing the identity of the material 
are mechanical agents. 

Changes in Temperature. — When stone is heated , it 
expands. When cooled , it contracts. If the heating or 
cooling is slow enough to change the temperature uniformly 
throughout the mass, the effect is slight. If, however, the 
rock is unequally heated or cooled, it produces the same 
sort of stress in the rock that is produced in a glass jar when 
hot fruit is poured into a cold jar. This stress caused by 
unequal expansion of different parts frequently breaks the 
rock just as it does the fruit jar. Both glass and rock are 
poor conductors of heat, and, therefore, when the surface of 
either substance is heated or cooled, the surface expands or 
contracts more rapidly than the interior, thus exerting a 
force that tends to pull the upper particles away from those 
below. This weakens the cohesion, and finally the outer 
layer is sheared off producing the effect shown in Figure 116. 
This is known as exfoliate weathering . 

Bare rocks on the top of Lookout Mountain, the Half 
Dome in the Yosemite, and many other mountains illustrate 
this effect. 

Every ledge of rock upon which the sun shines is subjected 
to this action to a greater or less degree; and when the daily 
range of temperature of the rocks is large, as it is in high 
altitudes, expansion and contraction is sometimes the most 
effective agent concerned in local weathering. 

When a laver of rock has been uncovered so as to receive 


268 


NEW PHYSIOGRAPHY 


the sun’s rays, as at the bottom of a stone quarry, the re¬ 
sulting rise in temperature expands the rock, producing 
tremendous lateral pressure which sometimes causes the 
rock to buckle and break. This pressure is increased in the 
daytime and diminished at night. These daily fluctuations 
in stress are effective in weakening the cohesion of the rock, 
thus assisting in weathering it, and the varying lateral 
pressure may materially aid in displacing the adjoining 
rock. 

In New York City a cement sidewalk 700 feet long and 
15 feet wide was completed in February. One warm day the 
following June the lateral pressure due to the high tempera¬ 
ture caused the sidewalk to buckle in three places, raising 
three miniature mountain ranges nearly a foot high across 
the walk. The stone was much broken at these places. It 
was repaired in July and has not since repeated the phe¬ 
nomenon. Why? 

When the Chicago and Northwestern Railroad was in 
process of construction, a portion of its line along the shore 
of Devil’s Lake, Wisconsin, passed over a large mass of very 
hard rock, quartzite, occupying a narrow space between a 
nearly vertical cliff of the same substance and the shore. 
After expending large sums of money experimenting with 
various kinds of drills, including the diamond drill, in an 
effort to remove the rock by blasting, the engineers were about 
to abandon the work when some one suggested that wood fires 
be built upon the rock and that when the rock was well 
heated a stream of cold water be thrown upon it. The plan 
was a success, and the quartzite was removed in this way. 
Farmers sometimes remove bowlders by this process. 

Freezing and Thawing. — Water is usually found in crev¬ 
ices and the minute spaces between the particles which 
compose the rock. When this water freezes, it expands and 
breaks the rock just, as water freezing in water pipes breaks 
the pipes. The effect upon the rock is the same as would 
be produced by driving minute wedges into, each space 


THE MANTLE ROCK 269 

containing water. This action is sometimes called the 
wedge work of ice. 

The process of freezing and thawing is more effective in 
weathering porous rocks than compact rocks; it is more 
effective in a moist than in a dry climate; and it is most 
effective in that moist climate where freezing and thawing 
alternate most frequently. 

The obelisk now in Central Park, New York City, stood 
for 3000 years near the mouth of the Nile in Egypt; yet 
when it arrived in the city, the inscriptions on it were finely 



Fig. 117.—Weathered Rock on the Summit of Pike’s Peak 


preserved. In a short time freezing and thawing had 
weathered it to such an extent that it became necessary to 
treat the surface of the obelisk with paraffine to fill the 
pores and keep the water out. 

Wind. — The sand blast, a device which blows a stream of 
sand against objects, is widely used as a means of cleaning the 
outside of stone buildings, removing rust from metals, etching 
glass, and similar processes. Wind-blown sand is a natural 
sand blast; it loosens particles from exposed surfaces of rock 
and adds them to the mantle rock. (See Figure 118.) 

Window panes in houses in certain localities on Cape Cod 
are abraded by wind-blown sand and their transparency 










270 


NEW PHYSIOGRAPHY 


destroyed; and in regions of strong winds pebbles are worn 
into triangular shapes and are even perforated. 

Plants and Animals. — The roots of plants find their way 
into cracks in rocks, and as they grow larger exert great 
pressure on the rock, often breaking off large pieces. Roots 
of trees growing near a city sidewalk frequently illustrate 
this action by raising or breaking the walk. 

Earthworms, moles, ants, and other animals living in the 



Fig. 118. — Effects of Wind-Blown Sand, Arizona 
By permission of Oliver Lippincott. 


ground bring much soil to the surface, exposing it to the air, 
and thus play an important part in changing insoluble 
minerals into the soluble form suitable for plant food. They 
also aid in the distribution of air and ground water through 
the tunnels and holes which they make. 

Gravity assists in weathering rock by removing loosened 
fragments from steep rock walls, thus exposing fresh surfaces 
to the air. 




THE MANTLE ROCK 


271 


Weathering below the Surface. — Certain kinds of weather¬ 
ing take place below the surface, but it is in general much 
less rapid than on the surface; indeed, one foot of impervious 
soil has frequently been found to have quite perfectly pre¬ 
served the polish and the scratches given the bedrock by 
continental glaciers. In porous mantle rock weathering 



Fig. 119 . — Oval Concretions 

Exposed by weathering of the weaker sandstone surrounding them. 
Near New Castle, Wyoming. 


certainly takes place at considerable depths. This is proved 
by the thick deposit of residual mantle rock which overlies 
some deposits of granite and other durable rocks. 

Residual Mantle Rock — Some portions of the mantle 
rock remain in the position in which they were formed, 
and such deposits are called residual mantle rock. All 
residual mantle rock is a product of the weathering of the 
bedrock below it and consists only of undissolved minerals 




272 


NEW PHYSIOGRAPHY 


and products of the chemical processes of weathering. The 
gravel and stones scattered through the deposit are all like 
the bedrock except as they show various stages of decompo¬ 
sition. The upper layers of residual mantle rock consist of 
smaller and more perfectly decomposed particles than the 
layers below them, because these upper layers protect to some 
extent those below them. There is usually a gradual in¬ 
crease in size and angularity of the fragments as we descend, 
as indicated in Figure 120. 

A change in the character of the bedrock is at once indi- 



Fig. 120. — Diagram of Residual Mantle Rock 


cated by a change in the nature of the mantle rock, and it is 
not usual to find large areas having the same kind of residual 
mantle rock. 

Deposits of Vegetable Matter. — During the last stages 
of the destruction of a pond or a lake, vegetable matter 
accumulates more rapidly than the other materials which 
fill them, and the swamp thus formed has a constantly 
increasing bed of but slightly decomposed plant fiber that 
is quite free from earthy matter and that burns well when 
dried. The trunks and limbs of trees, and every bit of plant 
fiber that falls into the water, and the roots of all the mosses 
and marsh grasses that grow in the water become a part of 


























THE MANTLE ROCK 


273 



the peat bed. Many ponds and marshes illustrate a stage 
in the formation of peat. Many peat bogs are found in 
the New England states, in New York, and in many other 
parts of the United States. The peat bogs of Ireland are 


Fig. 121. — The Dismal Swamp of Virginia 
Photo by Russell, U. S. G. S. 


well known and very extensive, one of them having an area 
of more than 600 square miles. 

The Dismal Swamp of Virginia (Figure 121) and the mil¬ 
lion-acre swamp of the Kissimmee Valley of Florida are 
examples of large deposits of a similar nature in the United 
States. In the Pacific Coast states are forests of giant trees, 



274 


NEW PHYSIOGRAPHY 


some of them more than 2000 years old, but no peat has been 
formed about their roots. Why? 

Peat beds, like residual mantle rock, have not been re¬ 
moved from the locality where they were formed. 

The mosses that help to form these deposits grow on the 
surface and die beneath, thus raising the surface so that it 
sometimes rises above the surrounding land or even climbs 
an adjoining hillside as in the “ climbing peat bogs.” 

Transported mantle rock is that which has been carried 
by some natural agent to the location where it is found. 
Its composition, as a rule, bears no relation to that of the 
underlying bedrock, and it is a mixture of fragments of many 
kinds of bedrock. Since certain agents which transport 
rock waste act over large areas, we sometimes find deposits 
of transported mantle rock of quite uniform composition 
and structure extending over thousands of square miles. 
The deposits of all large rivers illustrate this fact. 

Transportation of Mantle Rock. — Five agents are chiefly 
responsible for the transported mantle rock. 

1. Rivers. — Every muddy stream is actively engaged in 
the work of transporting mantle rock, and each stream is 
carrying a burden toward its mouth that is measured by the 
extent of the bottom lands along its valley and the depth 
of the transported mantle rock that forms the bottom lands. 
Mantle rock that has been transported by streams is called 
alluvial mantle rock. 

2. Glaciers. — The glaciers carry mantle rock slowly, 
but the size of the particle carried is not limited by the ve¬ 
locity, as it is in the case of rivers, and the total load that a 
glacier can carry is limited only by the amount that it can 
get. The greater part of the transported mantle rock in the 
northern United States and in northern Europe is glacial 
mantle rock. 

The largest known glacial bowlder (Figure 122) rests on 
top of the moraine of northern Long Island, on the Whitney 
Estate at Manhasset. It is a mass of the granite of the 


THE MANTLE ROCK 


275 


highlands of the Hudson. It is 54 feet long, 40 feet wide, 
and 16 feet high above ground. How much of it is under 
ground is unknown; but if it is of uniform height, it would 
load 65 of our 40-ton freight cars. 

3. Wind. — The presence of dust in the air is a familiar 
fact in every household; it settles on everything that air 
reaches. No building is so tall that the upper-story rooms 



Fig. 122. — Shelter Rock, Whitney Estate, Manhassett, Long Island 


never need dusting, and no mountain is so high that its 
snows are free from dust. 

In the Sahara a sand storm sometimes overwhelms cara¬ 
vans, and even when not fatal involves them in great con¬ 
fusion and danger. 

In the Missouri Valley during low water great clouds of 
sand and dust are picked up by the winds and carried many 
miles. In the arid regions of the Southwest it is claimed that 
the dust storms are as dangerous as the blizzards of the 
Northwest. 

Volcanic eruptions sometimes project great quantities of 
ash or volcanic dust (finely divided lava) into the air. The 



276 


NEW PHYSIOGRAPHY 


finer particles of this dust are carried great distances; indeed, 
it is believed that the dust projected into the air during the 
great eruption of Krakatoa in 1883 was carried to all parts of 
the earth and that some of it remained in the air for three 
years. This is probably the only way in which material 
from the land adds to the deposits forming in midocean. 

4. Gravity. — Under certain conditions gravity is the 
active agent that moves mantle rock down slopes. It is 
aided in this action by (1) ground water in clayey slippery 
soils, by (2) expansion and contraction due to changes in 
temperature; and in cold climates, by (3) the uplift due to 
freezing. If the motion is too slow to be seen, it is called 
hillside creep; if swift, it is a landslide or slump. 

Hillside creep sometimes moves trees, railroad tracks, 
and fence posts downhill, and sometimes even huge rocks 
weighing many tons. The trees and fence posts tend to 
slant downhill, because the surface of the ground moves 
faster toward the bottom than the soil a foot or two below 
the surface. 

5. Landslides. — In 1903 a large section of Turtle Moun¬ 
tain, Alberta, slid down into an adjoining valley destroying 
a part of the town of Frank. The mass of rock was about 
half a mile square and several hundred feet thick in places. 
Part of the rock was carried some distance up the opposite 
side of the valley. The time required for the movement was 
less than two minutes. 

A disastrous avalanche occurred February 27, 1910, in 
northern Idaho. It buried the mining towns of Mace and 
Burke, with great loss of life and destruction of property. 
On March 1, 1900, a train on the Great Northern Railroad 
was swept from the tracks by an avalanche which buried the 
track beneath a mixture of snow and earth. The accident 
occurred at Wellington, Washington, near the summit of the 
Cascade Mountains. 

6. Waves. — Between the breakers and the shore line 
water dashes up the beach from every incoming wave and 


THE MANTLE ROCK 


277 


carries so much of the beach sand with it that the water 
looks muddy. If the sand is white and free from clay, the 
water becomes clear at the instant that the shoreward 
motion ceases, to become muddy again as it gains velocity 
during its return. This latter motion follows the laws which 
govern motion down an inclined plane; it moves in the di¬ 
rection of the slope of the plane and increases its velocity 
at a rate which depends upon the slope of the beach. This 
return motion, the undertow , carries the finer particles of 
the beach deposit with it. 

Many ocean beaches are washed by waves of such vigor 
that all fine particles are carried out to sea by the undertow, 
leaving the sand white or gray and composed only of the 
more durable minerals. 

When the wave is oblique to the shore line, the to-and-fro 
motion of the water between the breakers and the shore is 
not along the same line as it is when the waves are parallel 
to the shore. This backward and forward motion transports 
the beach materials slowly along the shore. The amount 
of material transported in this way increases as the waves 
become more oblique and reaches a maximum when the 
wind is parallel to the shore. 

Deposition. — The agents that transport mantle rock 
deposit it as they lose carrying power and form physical 
features that differ so widely in shape and structure that, in 
most cases, the agent that transported a given deposit may 
be readily determined. 

1. River Deposits. — Streams assort the material that they 
deposit and wear off and round the corners of pebbles, thus 
giving their deposits the following characteristics ’. 

A. River deposits consist of layers of mud, sand, or gravel, quite 
perfectly assorted. 

B. As pebbles are carried downstream, they become rounder and 
smaller, and near the mouth of a long river they are composed chiefly 
of the more durable rocks. 


278 


NEW PHYSIOGRAPHY 


C. The deposits do not extend over large areas as do those of the 
ocean. 

D. The deposits have a nearly level surface. 

The principal physical features formed by river deposits 
are flood plains, deltas, and fans and cones, described in 
Chapter XX. 

2. Glacial Deposits. — Rock waste transported and de¬ 
posited by glacial ice is easily distinguished from that 
deposited by water. 

A. The rock fragments in them may vary from the finest particles 
to masses weighing many tons. 

B. The materials are unassorted and unstratified. 

C. Fragments of weak rocks like shale are found in them. 

D. The pebbles are largely angular. 

E. The surfaces of the stones are usually rough like freshly 
broken stone. 

F. Unlike river deposits, recent glacial deposits contain little 
decomposed rock. Even the smallest particles are ground rock rather 
than decomposed rock. 

G. Deposits made by glacial ice rarely have a smooth surface. 

Detailed treatment of glacial deposits will be found in 
Chapter XXI. 

3. Wind Deposits. — Dust and sand grains are supported 
in the air by irregular ascending currents, both convectional 
and forced. In the absence of such supporting currents, the 
larger particles settle quickly; but the weight of the smaller 
particles is so slight that it is nearly balanced by the re¬ 
sistance of the air to motion, and these particles settle slowly. 
The large particles will be deposited in one place and the 
small ones in another. This results in layers made up of 
particles which within certain limits are uniform in size and 
weight. The assorting is much less perfect than that of 
water, and the conditions causing deposit of a stratum of a 
given character are generally less permanent. The velocity 
of the wind is proverbially inconstant, and every change 


THE MANTLE ROCK 


279 


alters the size of particle deposited; but deposits formed 
by wind show distinct and characteristic stratification. 

Obstructions are effective in determining the location of 
the coarser particles to a somewhat greater extent even than 
they are in determining the location of snowdrifts, because the 
greater part of the sand is carried in the lower layers of 



Fig. 123. — Sand Dune Advancing over Trees 
Dune Park, Indiana. Note the steep slope of the lee side and the crescent-shaped front. 


the air. A rather larger proportion of such deposits, there¬ 
fore, will be found about obstructions. The deposit itself 
becomes an obstruction of increasing importance. Such 
hills of wind-deposited sand are called sand dunes. 

4. Sand Dunes. — The typical sand dune has a much 
more gentle slope on the windward side than on the leeward. 
This is true of even the smallest deposit, such as that formed 
about a chip; the sand grains carried by the air strike the 
chip, lose velocity, and drop or bound back, piling up on 



280 


NEW PHYSIOGRAPHY 


the windward side until the pile forms an inclined plane up 
which the wind can roll grains of sand. 

The leeward slope, Figure 123, owes its steepness to the 
fact that natural deposits each have a definite angle of re¬ 
pose or steepness which no deposit can permanently exceed. 

Standing beside a small dune when a strong wind is blow¬ 
ing, one sees the sand moving up the windward slope, stream¬ 
ing over the crest, and 
falling upon the leeward 
slope, which from time to 
time adjusts itself to the 
proper angle by miniature 
landslides formed where it has become too steep. The 
angle at which such a slope will come to rest is called the 
angle of repose and varies with the size and shape of the 
particles. 


Fig. 124. — Diagram of a Sand Dune 
Arrow shows wind direction. 


Occurrence. — Dunes are numerous along coasts, because sand 
is commonly found there. They are more likely to be formed by 
onshore than by offshore winds (why?); and they are more common 
on the east side of bodies of water in the prevailing westerlies and on 
the west side of similar bodies in the trade-wind belt than on the 
opposite sides. For example, dunes of great height occur on the east 
side of Lake Michigan, as at Grand Haven, Michigan, and very 
few are found on the west side of the lake. 

Dunes also abound in deserts and in the semiarid regions of the 
United States, sometimes reaching the height of several hundred 
feet. 

Migration of Dunes. — In regions subject to nearly constant winds 
the removal of sand from the windward side and its deposition on 
the leeward side causes the dunes to migrate slowly in the direction 
of the prevailing wind, sometimes overrunning highways, including 
railroads, destroying crops, and burying buildings or even forests. 
(See Figures 126 and 123.) 

Methods of Checking Progress of a Sand Dune. — Men often build 
fences in the path of a dune, just as railroads build fences along the 
tracks, to prevent the formation of snowdrifts on the tracks. These 
fences are mere temporary expedients. They may retard the prog- 



THE MANTLE ROCK 


281 


ress somewhat; and if the snow melts before the drift reaches the 
tracks, the fences have accomplished their object. Sand does not 
melt as the weather changes, and in the case of the sand dune the 
slight delay in no way lessens the destruction. 

Figure 126 tells the story of the fight made by the residents 
of Riggs, Oregon, against the advance of a sand dune that finally 



overwhelmed their village. The owner of this last house evidently 
brought all the fences used by the owners of houses previously 
buried to defend his house, but without effect. He was obliged to 
abandon it finally, and it looks.as if the railroad track in the fore¬ 
ground had also been abandoned. 

The most effective way of checking the progress of a dune is to 
cover it with vegetation. The beach plum, the huckleberry, the 
sand bur, and certain beach grasses have been used abroad for this 
purpose, and some of them have been used in this country. The 










282 NEW PHYSIOGRAPHY 

roots of the plants hold the sand, and each blade of grass and every 
twig serves as an obstacle that causes deposition on its windward 
side, thus preventing the removal of some of the sand. 

It is very difficult to get these plants to grow on a large dune 
where high wind blows almost continuously from one direction. 

The Loess. — In Kansas and other western states, in Europe, and 
notably in China, there are deposits called loess, consisting of par- 


Fig. 126. — The Last House in Riggs, Oregon 
Photo by Gilbert, U. S. G. S. 

tides larger than those of clay but smaller than those of sand. Their 
origin is in dispute, but there seems to be good evidence that a 
part of it, at least, is a wind deposit. It is without the distinct 
horizontal stratification of aqueous deposits and approaches con¬ 
solidated rock in its ability to stand with a nearly vertical face. 
Some deposits of loess are 1000 feet in thickness. 

5. Volcanic Dust. — In Kansas and Nebraska there are 
beds of volcanic dust three feet thick which cover large 
areas and which are hundreds of miles from either active or 




THE MANTLE ROCK 283 

extinct volcanoes. Pompeii was buried to a depth of about 
20 feet by such a deposit. 

6. Deposits by Gravity. — The most numerous of these 
deposits is the talus slope that forms at the foot of ledges of 
bare rock and that eventually covers the ledge with mantle 
rock. 

7. Shore Deposits. — The assorting action of the waves 
deposits layers of clay composed of particles of remarkable 



Fig. 127. — Stratified Clay 
Haverstraw, New York. Used chiefly for bricks. 


uniformity in size. Only the harder and more durable 
minerals remain on the beach, and as these grow smaller 
they are carried out to deeper water. This is why beach 
sand is chiefly quartz fragments. 

Useful Materials from the Mantle Rock. — In addition 
to the economic importance of the mantle rock as a whole, 
it is of much importance as a source of supply of clay, sand, 
gravel, marl, peat, and many materials used in the arts. 

Clay occurs in very large quantities, is widely distributed, 
is of various degrees of purity, and is suitable for many uses. 




284 


NEW PHYSIOGRAPHY 


The purest clay, kaolin, is used in manufacturing the better 
class of porcelain. The less pure varieties are used in making 
chinaware, pottery, terra cotta, tiles, drain tiles, and bricks. 
Fire clay, used in the manufacture of furnace and stove 
linings, Owes its ability to withstand high temperatures to 
the absence of lime and such alkaline substances as act as a 
flux. 

The clay products manufactured in the United States are 
valued at about $420,000,000 a year. This is more than 
$2,000,000 for every working day of the year. 

Sand is used in making glass, mortar and cement. It is 
also used as an abrasive as well as in the process of molding 
metals. Gravel is used in roofing, in concrete, and in road 
building. The total value of the sand and gravel used for 
these purposes in 1925 was $106,000,000. 

Marl is used as a fertilizer, in making certain kinds of 
bricks, and in making Portland cement 

The Soil 

Economic Importance. — The upper and fertile portion of 
the mantle rock is called soil. It differs from that below it, 
which is called subsoil, chiefly in the greater quantity of 
decaying animal and vegetable matter called humus and in 
the large number of bacteria which it contains. 

Agriculture has been the most important means of sup¬ 
port from the earliest times, and the progress of the early 
nations depended in a more marked degree even than that of 
modern nations upon the fertility of their soil and their skill 
in cultivating it. 

The Department of Agriculture estimates that the value 
of the direct and indirect products of the soil amounted 
to almost $18,000,000,000 in 1925. This does not in¬ 
clude the timber products not produced on farms. With¬ 
out the latter, the total is more than five times as great 
as that of all the minerals mined in the United States 
during 1925. 


THE MANTLE ROCK 


285 


Fertility. — Soils differ greatly in fertility from place to 
place, because of unlike composition and unlike texture. 

1 Composition. — All plants require nitrogen, potash, and 
phosphorus, and these elements of plant food must be natural 
constituents of the soil or must be supplied artificially to 
make the soil fertile. 

The soils of residual mantle rock contain only such of 
these elements as were in the rock from which they were 
formed. Granite and kindred rocks are usually rich in 
potash and deficient in phosphorus, though some of them 
contain the latter. A pure limestone usually contains an 
abundance of phosphorus, derived from shells, but is defi¬ 
cient in potash, and soil formed by its decomposition would 
be similarly deficient. A shaly limestone, like that at Tren¬ 
ton, New York, contains both phosphorus and potash. 
The famous “ Blue Grass Region ” of Kentucky has a soil 
formed by the decay of such a limestone. A pure sandstone 
contains neither phosphorus nor potash, and would form an 
unproductive soil, but sandstones containing many fossils 
produce a soil containing phosphorus. 

The unproductiveness of the sandstone soils in Kentucky 
is in marked contrast with the fertility of the adjacent “Blue 
Grass Region/’ and the sandstones of the Piedmont Region 
contrast in the same way with the fertile “Red Lands” 
adjacent to them. Transported soils are likely to be more 
fertile than residual soils, because the processes of transpor¬ 
tation tend to grind them finer, to mix the soils of different 
localities, and to increase the amount of organic matter in 
them. Such soils necessarily differ among themselves as 
the agents by which they were transported and deposited 
differ. 

The fertility of marine soils recently lifted out of the sea 
is low. There is a broad strip of land extending from New 
Jersey to Florida that has been dry land a comparatively 
short time. The soil is chiefly sand of little value for agri¬ 
culture. In a new land rain water naturally gathers in 


286 


NEW PHYSIOGRAPHY 


depressions in the surface, forming ponds and lakes. As 
time passes, the streams which bring down sediment and 
the plants which grow on the bottom tend to fill the lakes 
up, and thus they become marshes. 

In the Carolinas there are many such marshes, and on them 
most of the rice of the region is raised. These are lake soils 
or lacustrine- soils, as former lake bottoms are called. Some 
of them, like the great wheat-growing region in the valley of 
the Red River of the North, in North Dakota, and the fruit¬ 
growing region bordering Lake Ontario, in New York State, 
are among the most fertile soils of this country. 

Alluvial soils are very fertile, and the fertility may be 
renewed every time the river floods its basin. Examples of 
alluvial soils are found in every large river valley, such as 
the Nile, the Mississippi, and the Sacramento. 

Glacial soils are more variable in composition than alluvial 
soils, but they usually contain the required plant foods. The 
sandy glacial soils of Long Island and the clayey glacial 
soil of northern Ohio are both very fertile. 

Texture. — The physical condition of the soil is fully as 
important to its fertility as is the chemical composition. If 
the particles composing it are very small, the amount of 
water retained in the fine capillary passages between them 
will be large; and because of the high specific heat of water, 
the soil will warm slowly. Such soils are “ cold ” and “ late.” 

Fine-grained soils do not absorb so much of the rainfall as 
coarse-grained soils, and the run-off on the former is greater 
in proportion than on the latter type. The size of the 
particles composing the soil also determines the nature of 
the plant’s water supply, and hence the ability of the crop 
to withstand drought. If they are too large, water is not 
lifted a great distance by capillary action, and plants die 
when the water table is too far below the surface. If they 
are too small, water rises too slowly, with the same result. 

Direction of the Slope. — In the northern hemisphere, 
land sloping downward toward the south will produce earlier 


THE MANTLE ROCK 


287 


and larger crops than similar land sloping downward toward 
the north. 

Figure 128 A and B are cross-sections of an east and west ridge 
having a north slope and a south slope both of which make an angle 
of 30° with a horizontal line. The dotted lines, marked S, are the 
sun’s rays and those marked P are lines perpendicular to the slopes. 




A shows the sun’s rays when the sun’s altitude is 30°, about two 
hours after sunrise or two hours before sunset on the south slope. 
On such a ridge the south slope will receive about four hours more 
sunshine each sunny day than the north slope. 

B shows the angle of the noon rays of the sun on June 21 at New 
York City. Notice that the rays are more nearly perpendicular to 
the south slope than to the north slope. We learn in physics that 
the percentage of energy absorbed from sunlight depends upon the 
angle of the rays, and that the greatest amount is absorbed when the 






288 


NEW PHYSIOGRAPHY 


rays strike the surface at right angles. Therefore the south slope 
will be warmed faster during the time that the sun shines on both 
slopes, than the north slope is. 

It is the ultra-violet rays of sunlight that cause the de¬ 
velopment of vitamins and growth; hence a south slope in 
the northern hemisphere produces earlier and larger crops 
than a north slope. 

A similar but less important difference is observed between 
the crops raised on an east-facing slope and that raised on 
a west-facing slope, but in this case the result seems to be 
due to difference in the rates of radiation on the two slopes 
rather than to difference in the rates of absorption. 

Windward slopes in regions of steady winds have more 
cloudy weather than the leeward slopes, and the leeward 
slopes are drier. Thus the leeward slope will have earlier 
soil, but it may require irrigation or dry-farming; the 
windward slope will have more rainfall, but lack of sunshine 
may make the crops late. 

Water and Tillage. — Fertility of the soil requires some¬ 
thing more than plant food. It requires water in the right 
amount and at the right time; it requires heat; it requires 
air which must be distributed through the soil and must be 
renewed as it is exhausted; and finally, it requires tillage, 
which contributes mellowness, facilitates the renewal of the 
air supply, and conserves the supply of moisture. 

Types of Soil. — The common classification of soils as 
sands, loams, and days is based upon the physical structure 
or texture of the soil rather than upon its chemical composi¬ 
tion. It is true that coarse sands are usually composed 
chiefly of quartz grains and that clays contain a larger per¬ 
centage of kaolin than either sand or loam; but the dis¬ 
tinguishing characteristics such as plasticity and ability to 
hold moisture depend chiefly upon the size of the particles 
composing the soil. 

Sands are composed of particles between .04 inch and .002 


THE MANTLE ROCK 


289 


inch in size. Their distinguishing characteristic is their want 
of coherence when dry, and this characteristic is possessed 
equally by the sand composed of quartz fragments, with 
which we are all familiar, and by the sand found about 
coral islands, which is made up of fragments of coral 
and shells. 

Sandy soils are porous and well drained; they permit free 
circulation of the air, but are likely to suffer from drought. 
They are classed as “ early ” and “ warm ” soils; and if 
they are not too coarse, yield excellent crops of garden truck 
and potatoes. 

Clay is composed of particles less than .0002 inch in size. 
It is plastic when wet, shrinks on drying, but retains the 
form given it when plastic. It becomes impervious to water 
when puddled (worked with water to a thick paste). 

Clay soils permit very little circulation of the air. They 
are usually poorly drained and are therefore likely to be 
“ drowned ” in a wet season. They are less likely to suffer 
from drought than gravel or sand, but do not stand dry 
weather so well as loam. They are “ cold ” and “ late ” 
soils, but make good meadows. 

Loam is a mixture of sand and clay containing enough 
coarse particles to make the soil mellow and to permit 
free circulation of air. It also contains enough fine particles 
to facilitate capillary circulation, but not enough to make the 
soil sticky in wet weather. Loams are well drained and 
therefore stand wet weather well. They also stand dry 
weather well. 

Silt is the term applied to deposits of river-borne sedi¬ 
ments composed of particles between .002 and .0002 inch in 
size. 

Muck is a black soil formed in swamps and contains a large 
quantity of humus; hence it is rich in nitrogen. 

The following table shows the percentage of particles of 
various sizes to be found in some of the types of soil: 


290 


NEW PHYSIOGRAPHY 


Size Particles 

Barren Sand 

Coarse Sandy 
Loam 

Clay Loam 

Clay 

Sand, .04-.002 in. 

83.6 

75.6 

48.1 

7.6 

Silt, .002-.0002 in.... 

5.4 

7.2 

24.3 

32.2 

Clay, .0002 in. 

1.8 

11.7 

18.5 

42.2 


The sandy loam described in the table is an early and warm 
soil that is well drained and stands drought well. The clay 
contains so large a percentage of the finest particles that it 
is very wet during a rainy season and supplies water to 
plants so slowly that they would be parched during a drought. 

The Department of Agriculture at Washington publishes 
the following table of the percentage of each size of particles 
in typical soils for certain crops: 



Truck 

Corn 

Wheat. 

Grass 

Bright 

Tobacco 

Heavy 

To¬ 

bacco 

Barren 


Sandy 

Sandy 

Loam 

Clay 

Loam 

Clay 

Sand 

Clay 

Gravel, .08-.04 in. 








Sand, .04-,002 in. 

88.26 

45.86 

44.85 

11.68 

71.67 

10.04 

10.49 

Silt, .002-.0002 in. 

8.17 

40.90 

32.13 

23.69 

21.41 

44.98 

36.98 

Clay, 

.0002-.000,004 in. 

2.80 

10.10 

23.78 

51.75 

4.80 

35.24 

50.02 


The typical soil for vegetables or garden truck seems to 
be warm, sandy, and well drained; that for corn a sandy 
loam, and that for wheat a clay loam. 

QUESTIONS 

1. What two forces distribute water through the mantle rock? 

2. Show that the mantle rock tends to keep the flow of streams 
uniform. 

3. Why is spring water better for drinking purposes than surface 
water? 


























THE MANTLE ROCK 291 

4. How does the action of the chemical agents of weathering dif¬ 
fer from that of the mechanical agents? 

5. Under what climatic conditions is the action of freezing and 
thawing most effective in disintegrating rock? 

6. What kind of soil retards weathering below the surface? 

7. Dust is always present in the air, yet it is always settling. 
How is it supported in the air? 

8. Compare residual soils formed from granite with those formed 
from a pure limestone. 


CHAPTER XVII 


THE BEDROCK 

Composition. — With the exception of animal and vege¬ 
table deposits, the bedrock is composed of mineral sub¬ 
stances. In some rocks the minerals are in the form of 
crystals; other rocks are a fused mass of minerals, and still 
others consist of fragments of minerals or of products of 
the decomposition of minerals. 

Minerals. — A substance is a particular kind of matter. 
Rock is defined as a natural deposit . A mineral is a natural 
substance that is not obviously derived from the animal or the 
vegetable kingdom. We rarely find rocks composed of a single 
mineral. Granite, for example, always contains two minerals 
and may contain several more. 

Rock-making Minerals. — As the economic value of any 
rock depends upon the properties of the minerals that it 
contains, whether it is used as a building stone or in chemical 
processes in the industries, we will briefly consider the 
properties of the principal rock-making minerals. 

There are three groups of minerals that are the principal 
constituents of three-quarters of the known bedrock. They 
are: 

1. The silica group, of which quartz is an example. Silica is a 
compound formed by the chemical union of the elements silicon and 
oxygen. 

2. The silicates , of which feldspar and mica are examples. Sili¬ 
cates consist of silica combined with one or more metals. 

3. The calcareous minerals, of which calcite is an example. Cal¬ 
careous minerals consist of the element calcium combined with carbon 
dioxide or other substances 


292 


THE BEDROCK 


293 


Properties of Minerals. — The most exact method of 
distinguishing minerals is by chemical analysis, but the 
common minerals may be distinguished by a careful com¬ 
parison of their physical properties, such as shape of crystal, 
color, hardness, luster, 
and cleavage. 

Cleavage. — Some 
minerals break or split in 
certain directions more 
readily than in others, 
leaving flat, shiny sur¬ 
faces that are parallel to 
each other; and the 
cleavage, as this quality 
is called, is often an aid in 
distinguishing one mineral 
from others. Mica, for 
example, easily splits at a 
certain angle with the 
axis of the crystal into thin sheets of uniform thickness with 
highly polished surfaces; but mica cannot be thus split at 
any other angles. Some varieties of feldspar split in two direc¬ 
tions approximately at right angles to each other, as shown 
in Figure 129. 

The front of the specimen is split along a number of planes 
all of which are parallel to each other and which make a 
definite angle with the axis of the crystal. The bottom of 
the specimen shows another set of planes also parallel to 
each other and approximately at right angles to the first set. 

In other words, feldspar has two cleavages at right angles 
to each other. It may be split easily in any part of a crystal, 
along either of these cleavage planes; but it is broken with 
difficulty in any other direction, leaving rough waxy sur¬ 
faces. 

Calcite, Figure 132, splits in three directions at oblique 
angles. 



Fig. 129. — A Group op Prismatic Quartz 
Crystals 

From Dauphiny, France. Photo by American 
Museum of Natural History. 



294 


NEW PHYSIOGRAPHY 


We speak of these minerals as having one, two, or three 
cleavages. 

Broken pieces of some minerals, like obsidian, Figure 137, 
break with smooth, shiny surfaces; but the surfaces are 
curved, resembling the surface shell. These minerals are 
said to have a shell-like fracture. 

Quartz, glass, and hard coal are substances that have this 
sort of fracture. 

Hardness. — It is customary to compare minerals with a 
standard set of test minerals called a scale of hardness. 
Arranged in the order of their relative hardness, beginning 
with the softest, they are: (1) talc, (2) gypsum, (3) calcite, 
(4) fluorite, (5) apatite, (6) orthoclase feldspar, (7) quartz, 
(8) topaz, (9) ruby, (10) diamond. A mineral that will 
scratch feldspar but not quartz is said to have hardness 7. 

Luster. — Girls can tell whether a friend has a silk or a 
satin dress on clear across the assembly room. They recog¬ 
nize the luster of the satin. For our purposes we will need 
to use only three grades of luster. If a mineral has a dull 
surface like a woolen cloth or a piece of wax, we say it has 
a waxy luster. If it shines like quartz, it has a glassy luster; 
and if it looks like a piece of polished metal, it has a metallic 
luster. 

Quartz. — This is the hardest and most durable of the 
common minerals. It is brittle, and the broken surface is 
uneven and curved like the surface of a shell. It melts with 
great difficulty, and very few chemicals affect it. When 
crystallized, it is a six-sided prism terminating at one or 
both ends in a six-sided pyramid. It is usually colorless 
and transparent, but it may be opaque, white, or 
colored. 

Occurrence. — Quartz is the chief constituent of quartzite, 
sandstone, and common sand; and is an important constit¬ 
uent of granite, gneiss, and mica schist. It is also a valuable 
ingredient in many soils. (See page 288.) It is the most 
abundant mineral in the world. 


THE BEDROCK 


295 


Uses. — It has been so difficult to work quartz and has 
required so high a temperature to melt it that the high cost of 
production limited its use in manufactured form to small 
lenses and prisms used in optical experiments. Fused quartz, 
however, which formerly cost from six to ten times its weight 
in gold, is now being made by improved processes that will 
doubtless lead to its use for many purposes. It is the most 
transparent substance known. Light follows a curved rod of 
fused quartz as electricity 
follows a copper wire, or as 
water follows a hose. It is 
as if the quartz rod were 
a hole in the air through 
which light waves might 
pass more easily than 
through the air. Light 
rays pass through a meter 
rod of quartz with only 8 
per cent loss. In a similar 
rod of the best optical 
glass the loss is 35 per 
cent, and in ordinary glass 
the loss is 65 per cent. 

A considerable number of the smaller crystals are cut in 
imitation of diamonds and sold under the name of rhine¬ 
stones. It is used in a limited way for prisms, crystal balls, 
etc. Spectacle lenses were formerly made of quartz. 

Several colored varieties of quartz and several other mem¬ 
bers of the silica group are used as gems. The list includes 
amethyst, jasper, agate, carnelian, opal, onyx, sardonyx, and 
chrysoprase. 

When pure quartz sand is melted with lime and soda ash 
(lye), it forms glass, such as is used for windows, bottles, etc. 
Much quartz sand is also used in the manufacture of porce¬ 
lain. To make the latter, fine sand is mixed with pure clay 
and ground feldspar to produce a glaze. Water is added to 




296 


NEW PHYSIOGRAPHY 


form a plastic mass which is molded and u fired,” i.e., held 
at a high temperature in a furnace for many hours. It is 
then coated with a mixture of fusible minerals like feldspar 
and fired again. This produces the glazed surface familiar 
to us on our table porcelain. 

Flint, another member of the silica group, was of great 
importance to prehistoric man because of the sharp cutting 
edge of broken pieces. From this substance he fashioned his 
cutting implements such as knives, awls, spear heads, and 
arrow points. 

Later the flint and steel were used to produce a spark to 
kindle fires. Still later the flintlock musket was used. 

As before stated, quartz is an example of the silica 
group. 

The Feldspars. — These minerals are silicates. They 
occur in a variety of colors, commonly pink, gray, yellow, 
or white; but occasionally they are blue or iridescent. 
They are next to quartz in the scale of hardness, but easily 
distinguished from quartz by their cleavage surfaces, of 
which there are two. In certain varieties the cleavage 
planes are at right angles to each other, as in Figures 130 
and 131. 

When exposed to moist air containing carbon dioxide or 
to infiltrating water containing carbon dioxide or acids, its 
luster is quickly lost, and it soon crumbles into a soft clay, 
called kaolin. Because of its ready cleavage and its lack 
of permanence under natural conditions, feldspar is not a 
durable mineral, and most of the clay and the mud rocks 
of the earth are chiefly products of its decomposition. 

Occurrence. — The feldspars are widely distributed in the 
rocks. They are important constituents of granite, gneiss, 
and many lavas. Feldspar is extensively quarried in New 
York, Pennsylvania, Maine, Connecticut, Maryland, and 
Canada. 

Uses. — Several varieties of feldspar are used in making 
the so-called agate ware and as a cement in making grinding 


THE BEDROCK 


297 


wheels of carborundum and of emery. They are also used 
as an ingredient of certain scouring soaps. 

The feldspars and kaolin are used in the production of 
porcelain. 

Some varieties of feldspar contain as much as 17 per cent 
of potash, which will make them valuable as a source of this 
much-needed fertilizer when a cheap process of extracting 
it is perfected. 

The Micas. — Another important rock-making silicate is 
familiar to every one in the misnamed “ isinglass.” Its 



Fig. 131. — A Group op Calcite Crystals 
Photo by Arey. 


proper name is muscovite or white mica. Its most important 
properties are: its perfect cleavage into very thin elastic 
sheets or flakes having flat surfaces and a high luster; 
its high electrical resistance, and its ability to withstand 
high temperatures. It is so soft that it may be scratched 
with the fingernail. It is between 2 and 2.5 on the scale 
of hardness. Black mica, having similar properties and 
composition, is called biotite. 

Uses. — Mica is much used as an insulator in electrical 
machinery and fixtures. No other substance serves this 
purpose so well. It is used in stove doors and lamp chimneys 




298 NEW PHYSIOGRAPHY 

because of its ability to withstand heat. Its high luster has 
led to its use to some extent in decorations, and it is some¬ 
times used as a lubricant. 

Occurrence. — Mica is the chief constituent of mica schist 
and an important constituent of granite and gneiss. It is 
found in some sandstones. 

Calcite. — When pure, calcite is a transparent, colorless 
mineral. Impure specimens show a variety of colors blue, . 

yellow, red or pink, and 
gray. It usually forms a 
pointed crystal that some¬ 
what resembles canine 
teeth and leads to its be¬ 
ing called dogtooth spar 
by the miners. Figure 131 
is a photograph of a group 
of such crystals. 

Calcite crystals cleave 
very perfectly in three 
directions at oblique 
angles with each other. 
When crushed by a blow 
the fragments are very 
perfect rhombohedrons. 
(See Figure 132.) 

It is much softer than 
quartz and is easily 
scratched with a knife. 

Its effervescence with cold dilute acid and the fact that 
objects seen through a transparent specimen appear double 
easily distinguish it from other common minerals. 

It is dissolved by water containing carbon dioxide in solu¬ 
tion; therefore rocks containing calcite are disintegrated by 
rain water, which insinuates itself along the cleavage planes. 
Calcite rock is thus less durable then feldspar or quartz. 



Fig. 132. — A Cleavage Specimen of 
Calcite. A Rhombohedron 

Photo by Arey. 




THE BEDROCK 


299 


Occurrence . — Calcite is one of the most abundant min¬ 
erals. It is the chief constituent of marble and of all lime¬ 
stones, including chalk and travertine, and also of much of 
the ooze found on the floor of the ocean. 

Uses. — Transparent crystals of calcite are used in optical 
instruments. They are becoming scarce and very valuable. 



Fig. 133. —- Stbatified Rock near Engineer Mountain, California 
Photo by U. S. G. S. 


Gypsum. — This mineral is a compound of calcium sul¬ 
phate and water. 

Properties. — Its crystals are transparent and colorless, 
with one perfect cleavage that often causes them to be mis¬ 
taken for calcite and sometimes for mica. Gypsum is not 
affected by acid. It is number 2 in the scale of hardness and 
can be scratched with the fingernail. The gypsum crystals 
do not show a double image and have not the three cleavages 
of calcite. 

Mica is harder than gypsum and cleaves into thinner 



300 


NEW PHYSIOGRAPHY 


sheets. The sheets of gypsum are not elastic like those of 
mica and seem brittle because of an imperfect perpendicular 
cleavage. 

Alabaster is a fine-grained massive gypsum streaked with 



Fig. 134. —'Several Thousand Feet of Stratified Rock, 
Grand Canyon of the Colorado 


The upper layers are not parallel to the lower ones, showing that the lower layers were 
titled before those forming the buttes were deposited. Copyright by Oliver Lippincott. 


various pale colors. Rock gypsum is usually an impure mas¬ 
sive variety. 

Uses. — Crystals of gypsum are used in optical instru¬ 
ments. Alabaster and the purer deposits of rock gypsum 
are used for ornaments and as a substitute for marble in 
buildings. 



THE BEDROCK 


301 


Structure of the Bedrock. — As we approach a rock cliff, 
the most conspicuous feature, in most cases, is the arrange¬ 
ment of the rocks in layers, like a great layer cake; as in 
Figures 133, 134, and others. 

The layers may vary in color or in kind of rock, and may 
vary in thickness from a small fraction of an inch to many 
feet, or there may be many layers of the same kind of rock. 
They seem to be parallel to each other, and if they have not 
been disturbed, are nearly horizontal. Rocks arranged in 
layers, or strata, as they are often called, are said to he stratified. 
Most of the surface rocks of the continents are stratified, 
though in certain localities we find rocks that seem to have 
been formed in great masses, sometimes hundreds of feet 
in thickness, practically uniform in color and composition 
and without stratification as in Figure 135. 

These are the unstratified or massive rocks. They are 
more apt to appear on the surface in mountainous regions, 
but are found everywhere below the stratified rock when we 
bore through the latter. The bedrock consists of a great 
mass of unstratified rock which is covered in most places by 
the beds of stratified rock. As a rule, the unstratified rock 
is reached by borings less than a mile deep, but in some places 
the stratified rocks are much thicker than that. In the 
Colorado Canyon, Figure 134, more than 8000 feet of con¬ 
secutive stratified rocks are exposed at one point, but at the 
bottom of the canyon unstratified rock is found. 

Origin of Bedrock. — Portions of the bedrock show that 
they assumed their present form on cooling from a molten 
state and are therefore called igneous rocks. The great mass 
of these rocks underlies all other kinds and is perhaps the 
original rock of the earth. Modern lavas, however, are ig¬ 
neous rocks and may be the youngest deposits in their vicin¬ 
ity, but they form a relatively unimportant portion of the 
whole. 

Other portions of the bedrock accumulated as sediments 
or deposits in bodies of water. They are called aqueous rocks 


302 


NEW PHYSIOGRAPHY 



Fig 135 —Unstratified Rock, Yosemite Valley 
Copyright, Underwood & Underwood 



Fig. 136 . — Distribution of Sedimentary, Metamorphic, and Igneous Rocks 

in the United States 


THE BEDROCK 


303 





























































304 


NEW PHYSIOGRAPHY 


or sedimentary rocks, and are always stratified. Most strati¬ 
fied rocks accumulated in this way. A small group, however, 
accumulated on land. 

Sedimentary rocks are made up of materials derived from 

former rocks; hence they 
are called derived rocks or 
secondary rocks. 

Other parts of the bed- 
rock have been so 
changed from their 
former igneous or sedi¬ 
mentary character as to 
give the rock new prop¬ 
erties. These are called 
metamorphic rocks. 

Surface Bedrock of the United States. — In the white 
area in Figure 136 the surface layers of bedrock are of sedi¬ 
mentary origin. In a few minor regions included in the white 
area, the mantle rock is so thick that the character of the 
underlying bedrock has not been established. 

In the shaded areas the bedrock is metamorphic. Note 
the great mass of metamorphic rock about Hudson Bay. A 
second mass extends from Maine to Alabama, along the Ap¬ 
palachian Mountains, and a third along the western edge 
of the Mississippi Valley. These regions must have been dry 
land while the Mississippi Valley was receiving its thick de¬ 
posit of sediments; otherwise these regions also would have 
received sedimentary deposits. 



Trace the outlines of the dry land of North America when these 
sedimentary rocks were forming and mark it “ North America Many 
Years Ago.” 

The areas marked v v v are of igneous origin. Note the great 
area in Washington and Oregon with a great lava flow extending 
southward several hundred miles into California and a second one 
extending across Idaho. 



THE BEDROCK 


305 



Igneous Rocks 

Obsidian. — Some lavas come to the surface in a very 
fluid condition and, on cooling quickly, form a glassy mass 
in which it is impossible to distinguish separate crystals of 
minerals. Obsidian is such lava. It contains 70 per cent 
or more of silica and occurs in green, brown, and black. In 
thin sections it is translucent with a shell-like fracture and 
with the same hardness as glass. 

It is found in the upper part of many lava flows in Tene- 


Fig.. 138. — Implements Made of Obsidian 

rifle and Iceland. It also forms Obsidian Cliff in Yellow¬ 
stone Park. 

Obsidian was used by the North American Indians for 
arrowheads and spearheads when flint was scarce (Fig¬ 
ure 138). It is now used with an electric light behind it 
to produce “ stage fire.” 

Pumice. — This is a lava that was impregnated with steam 
when it cooled, forming a porous mass that floats on water. 




306 


NEW PHYSIOGRAPHY 


Most of the commercial article comes from the volcanic 
islands of the Mediterranean. It is used instead of sand¬ 
paper by piano finishers and automobile painters to make a 

smooth surface. In its 
powdered form it is a 
part of many toilet 
preparations. 

Volcanic Ash or 
Dust. — When a lava, 
impregnated with 
steam under great pres¬ 
sure, is thrown into 
the air by a volcano, 
the sudden expansion 
of the steam will burst 
the walls of the cells 
enclosing it so that it 
falls as ash if the parti¬ 
cles are coarse, or as 
dust if they are very small. Enormous deposits of ash have 
accumulated about many volcanoes, destroying valuable 
lands and many cities. Pompeii was buried under a mass of 
volcanic ash from which it is still being excavated. The sea 
was covered so deeply with pumice and ash that ships found 
navigation difficult in that part of the Pacific Ocean near 
the volcano Krakatoa after its great eruption. The volcano 
Katmai in Alaska probably ejected a larger mass of ash in 
1903 than had ever been ejected before by any volcano. 
Though thick deposits of volcanic ash are always powerful 
enough to destroy whole forests, farms, and towns, these 
deposits are useful later, when weathered, in supplying agri¬ 
culturists with a very fertile soil. 

Granite. — This is an igneous rock that cooled slowly be¬ 
neath a thick rock blanket, thus permitting the growth of 
large crystals. Granite always contains quartz and feldspar, 
and most granites contain in addition one or both of the 



Fig. 139. —- Fine-Grained Granite prom 
Westerly, Rhode Island 

Photo by Arey. 




THE BEDROCK 


307 



common micas. The crystals of the component minerals are 
often so large that the mineral can be recognized without 
the aid of a lens. They are irregularly distributed through the 
mass. Granite is one of the most durable rocks. 

When granite is found on the surface, it is evident that the 
rock blanket that originally covered it has been worn away 


Fig. 140. — A Granite Quarry 

or that the earth’s crust has been uplifted and the strata 
overturned by some force. 

Granite is quarried extensively for monuments at Quincy, 
Massachusetts; Barre, Vermont; and Westerly, Rhode Is¬ 
land. (See Figure 140.) There are many other quarries 
along the Atlantic coast of the United States from Maine to 
Georgia that supply it for building material. A fine granite 
is found at Montello and another at Wausau, Wisconsin. 

Granite forms the central mass of many of our western 
mountains and is quarried at a number of places in the 
Sierra Nevada Mountains in California. 





308 


NEW PHYSIOGRAPHY 


Uses. — The annual production of granite stands second 
only to that of limestone. It is used for monuments, 
buildings, pavements, and when crushed, for road making. 

As it usually occurs in large masses without seams or 
layers, it is possible to obtain large blocks, often many times 
as large as those shown in Figure 140. Sometimes, however, 
granites have been so badly fractured by the forces that 
bend and stretch the earth’s crust that no large pieces can 
be obtained. Figure 114 shows a granite ledge thus broken. 

Fine-grained Igneous Rocks. — There are many varieties 
of igneous rocks in which the individual crystals cannot be 
distinguished without a microscope. When light-colored, they 
are called felsites, and when dark-colored, basalts. These 
rocks evidently cooled so quickly that the crystals did not 
have time to grow large as they did in granite, perhaps be¬ 
cause they were covered by a thinner rock blanket while 
cooling. 

The basalts form a large and important class of igneous 
rocks that covers thousands of square miles of the earth’s 
surface. They are compact rocks of a dark gray, dark green, 
or black color. Since they are* heavy and not easily broken 
or powdered, they therefore make durable and comparatively 
dustless roads. 

The rock of the palisades of the Hudson River and the 
mountain ranges parallel to them in New Jersey are basalt. 
The term trap is used for basalt and any other heavy igneous 
rock of similar texture. 

Basalt is quarried in the ranges mentioned above and also 
in Connecticut and Pennsylvania. It is probably unequaled 
for road metal, as the broken stone used in road making is 
called. 


Sedimentary Rocks 

In every body of water, the solid particles blown into the 
water by winds or carried in by streams, glaciers, or other 
agents settle to the bottom as sediments. While conditions 


THE BEDROCK 


309 


under which they are deposited remain constant, they accu¬ 
mulate in nearly horizontal layers of particles which are uni¬ 
form in both size and weight. But when the source of supply 
or other conditions are changed, the particles also change 
materially and form strata of different kinds. 

All sedimentary rocks are stratified. 

Sedimentary rocks are found in the mantle rock as beds of 
loose sand, gravel, clay, shells, and marl; and in the bedrock 
as consolidated conglomerate sandstone, shale, and limestone. 



Fig. 141 A . — The Oneida Conglomerate with Rounded 
and Flattened Pebbles 

Photo by Arey. 


Many of the sedimentary rocks retain the original horizon¬ 
tal position of their layers. This is particularly noticeable 
in plains and plateaus, but in mountainous regions the layers 
are often quite steeply inclined, and sometimes nearly vertical 
or even overturned. The layers of rock in the lower part cf 
Figure 134 are inclined. 

Mechanical Sediments. — The material forming these 
sediments was derived directly from older rocks and consists 
of their more durable minerals. 

Sandstone. — Because of the great abundance of quartz 
and because it is one of the most durable of the rock-forming 
minerals, most of the grains of a sand bed are quartz. Sand¬ 
stone is a cemented bed of quartz sand. The cement is 
usually silica, but sometimes it is iron ore or even calcite. 



310 


NEW PHYSIOGRAPHY 


Properties. — Sandstone is readily recognized by the 
following properties: 

1. Its rough feel. “It feels 
like a grindstone.” 

2. Its porosity. (Dip a glass 
rod in water and touch a piece 
of dry sandstone with it. Note 
whether the drop of water is 
absorbed by the stone at once 
or remains on the surface for 
some time.) 

3. The visibility of its par¬ 
ticles. 

4. The fact that it is harder 
than glass. 

5. Acid does not affect it. 
Sandstone occurs in a 

number of colors, but the 
reds and yellows are most 
common. 

Because of its porosity, it 
is both the principal water¬ 
bearing rock and the oil¬ 
bearing rock. A liquid may 
flow through the cracks in an impervious rock as a stream, 
but a porous rock is saturated and can supply great quan¬ 
tities of the liquid. 

In temperate latitudes porous rocks weather more quickly 
than others through repeated freezing and thawing of 
absorbed water; but in sandstone like that found at Potsdam, 
New York, in which the pores are well filled with siliceous 
cement, freezing and thawing have little effect. 

Uses. — Sandstone from Berea, Ohio, makes superior 
grindstones and whetstones, and a Michigan sandstone is used 
for the same purpose. The Shawngunk grit, a sandstone 
consisting of very fine particles, is used to make millstones 
for grinding paints and cereals. 



Fig. 141 B. — A Conglomerate with 
Smoothed and Rounded Pebbles 

Photo by Arey. 



THE BEDROCK 


311 


A sandstone composed of particles invisible to the naked 
eye is used for whetstones, or oilstones, as they are called 
when oil is used to prevent particles of steel adhering to the 
stone. Oilstones are much used for sharpening small tools. 
The best known deposits of sandstone suitable for this pur¬ 
pose are found in Arkansas, Indiana, and Ohio. Sandstone 
is quarried for building purposes in Arkansas, Indiana, New 
York, Ohio, and Wisconsin. 

Conglomerates. — A conglomerate is a cemented gravel bed. 
The pebbles that form the conglomerate are eroded frag¬ 
ments that belonged to a previous generation of rocks. Some 
of them were rolled over and over so that the corners were 
knocked off and the surfaces were smoothed until they be¬ 
came quite perfect spheres or spheroids. Figure 14 IB is 
such a conglomerate. 

When rounded pebbles reach 
the ocean or other large body 
of water where wave action is 
vigorous, the to-and-fro motion 
of the water slides the pebbles 
over the sands and after a time 
flattens them. The five peb¬ 
bles taken from the conglom¬ 
erate and shown in Figure 141A 
have thus been flattened. 

The pebbles of each of these 
conglomerates consist only of 
the more durable minerals, 
chiefly quartz, indicating that 
erosion had reduced the less 
durable minerals to particles 
small enough to be carried away. 

Certain conglomerates are composed of durable minerals 
only, but have a subangular form. Figure 142A is a quartz 
conglomerate with subangular pebbles and a siliceous cement, 
and Figure 1425 a conglomerate with similar pebbles and an 



Fig. 142A. — Rock City Conglo¬ 
merate 

Photo by Arey. 



312 


NEW PHYSIOGRAPHY 


iron cement. Figure 142(7 is a bottle filled with pebbles 
sifted from the Long Island terminal moraine. 

Shale. — The finest particles of rock waste settle to the 
bottom in quiet water, farther from the shore than the sand 
deposits. When consolidated the beds thus formed are 
called shales or mud stones. 



Fig. 142 B. — A Conglomerate with Subangular 
Pebbles 

Photo by Arey. 


Properties . — (1) When moistened, shale has the odor of 
wet clay. 

(2) It splits readily into thin layers parallel to the bed¬ 
ding planes. 

(3) Water does not pass through unbroken shale. 

(4) It is so soft that it may be scratched with the fingernail. 

(5) It weathers quickly. (6) It is not affected by acid. 
(7) It is of many colors — perhaps gray is the most 

common, — but there also are black, green, and red shales. 

Uses. — Though worthless for building purposes, shale is 
used in making Portland cement, terra cotta, and bricks. 





THE BEDROCK 


313 


Some of the black shales contain valuable oils of the petrol¬ 
eum group, which may be recovered by distillation. Such 
shale will doubtless be the principal source of petroleum in 
the future. 

Occurrence. — Shale is a very abundant rock and forms 
an important part of most ledges of sedimentary rock. 

Chemical Sediments. — The materials forming deposits of 
this class are recovered from solution in water by natural 
chemical processes. 

The principal processes involved are: (1) concentration 
of the solution, (2) cooling or diminishing pressure, (3) loss 
of carbon dioxide. 

These processes result in the 
precipitation of solids that some¬ 
times form beds of very pure 
chemical compounds. 

It is estimated that a cubic 
mile of average river water 
carries to the ocean the follow¬ 
ing weights of soluble min¬ 
erals: limestone, 326,710 tons; 
gypsum, 34,361; magnesium car¬ 
bonate, 112,870; saltpeter, 

26,800; common salt, 16,600. 

If a cubic mile of such water 
should be evaporated the above 
weights of the minerals men¬ 
tioned would be deposited in 
solid form. 

There are many deposits of bedrock that must have been 
formed in this way. They do not consist of a mixture of all 
the dissolved minerals as would be the case if all of the min¬ 
erals were deposited at the same time. They consist instead 
of separate layers of quite pure minerals. 

The principal materials deposited in this way are gypsum, 
rock salt, iron ore, geyserite, and certain kinds of limestone. 



Fig. 142 C . — Glacial Pebbles 


Photo by Arey. 





314 


NEW PHYSIOGRAPHY 


Rock Salt. — Large deposits of rock salt are found in 
New York, Michigan, Louisiana, and Texas; less important 
ones in Utah and California. The thickest deposit of chemi¬ 
cal sediments in the world is at Stassfurt, Germany. It is 
4794 feet thick. 

The section of New York State west of Syracuse and 
south of the Erie Canal has a deposit that in places reaches 
a thickness of more than 300 feet. Most of the mines and 
salt wells now worked in New York State lie in a belt about 
40 miles wide, just south of the canal, because the salt beds 
are nearer the surface in this section. Along the southern 
line of the state the salt beds are 3000 feet or more below 
the surface. 

The Saginaw Valley in Michigan supplies much salt from 
its natural brines, and there are also great deposits of rock 
salt along the rivers connecting Lakes Huron and Erie. 

Formation. — Rock salt is found in deposits too large to 
have been deposited by a salt spring and of such thickness as 
to have required either an enormous depth or a continual 
supply of salt water. Salt rivers flowing for many centuries 
into a lake might accumulate a bed of salt several feet in 
thickness, but beds several hundred feet in thickness are 
improbable. 

It is more likely (1) that the climate in New York State 
and Michigan was very arid when the salt was forming, mak¬ 
ing evaporation rapid, and (2) that a shallow. arm or arms 
of the sea covered the regions where salt is found and that 
the rapid evaporation kept the surface of the water in the 
arm constantly below sea level, thus causing a constant 
flow of salt water into the area. 

More than nine-tenths of a body of sea water must be 
evaporated before salt crystals begin to form. 

Uses. — Salt is used in the manufacture of a number of 
important chemicals, including baking soda, washing soda, 
soda ash or common lye, and hydrochloric acid. It is also 
used in the process of extracting gold from its ores, in pre- 


THE BEDROCK 


315 


serving meats and fish, and in making butter and cheese as 
well as for freezing mixtures and domestic purposes. 

Gypsum. More than half of the gypsum mined in the 
United States comes from New York, Michigan, and Iowa. 
There are undeveloped beds of great thickness that cover con¬ 
siderable areas in some of 
our western states. Okla¬ 
homa is called the gypsum 
state. (See Figure 279.) 

Gypsum is associated 
with rock salt in the 
deposits of southwestern 
New York. There are 
important deposits at 
Grand Rapids and Ala¬ 
baster, Michigan, and 
near Fort Dodge, Iowa. 

Formation. — Gypsum 
begins to be deposited 
when 37 per cent of a 
body of sea water has 
been evaporated. 

Uses. — Slabs of gyp¬ 
sum are polished and used as a substitute for marble in wain¬ 
scot tings. As it is less soluble than marble, it retains its polish 
rather longer than marble does; but it is softer than marble, 
therefore more easily dented and cut. About 15 pounds of 
gypsum is used in making a barrel of Portland cement. It 
is also used in making plaster of Paris and fertilizers. 

Iron Ore.— Certain kinds of iron ore are found in stratified 
layers among sedimentary rocks. One such ore bed extends 
from western New York to near Albany and thence south¬ 
ward to Alabama. The same ore also occurs in Nova Scotia, 
Vermont, and elsewhere. It often contains marine fossils. 

Formation. — Iron forms two classes of compounds, one 
which dissolves readily in water and another which is insolu- 


■ 

;a ' 


$«%*££***$, 
y&A 

c#4 


.V- - ':X. 'S. 

.... c* 



Fig. 143. — Calcareous Tufa, Mumford, 
New York 

Photo by Arey. 




316 


NEW PHYSIOGRAPHY 


ble. Both of these compounds are widely distributed, particu¬ 
larly in the igneous rocks. The soluble compounds have but 

little effect upon the color 
of rocks containing them, 
but the insoluble com¬ 
pounds color them red 
or yellow. 

The soluble compounds 
of iron in the rocks and 
soil are dissolved by the 
ground water and carried 
to the streams. When 
such water flows into a 
body of shallow water 
containing decaying 
vegetable matter — be it 
swamp, lagoon, lake or 
even a part of the sea — 
the vegetable matter 
changes the soluble iron 
compounds into the insoluble form, and it is precipitated as 
a sediment. Beds of this kind are now forming, and in the 
past have formed deposits 
many feet thick. The ore 
thus formed is not a rich 
ore, but it is used quite 
extensively in the produc¬ 
tion of iron and steel. 

Travertine. — This is 
the general term applied to 
limestones deposited by min¬ 
eral springs or lakes of hard 
water. (See page 376). The conditions that cause the deposi¬ 
tion of minerals by ground water are discussed on page 377. 

When travertine is deposited rapidly on vegetation grow¬ 
ing about springs, it forms a soft porous mass with holes 




Fig. 144. — Banded Travertine, Suisttn, 
California 


Photo by Arey. 





THE BEDROCK 


317 


in it through which grasses grow, and often has impressions 
of leaves and twigs in it. It is called calcareous tufa or some¬ 
times petrified moss. (Figure 143.) 

Deposits of calcareous tufa often cover many acres and 
have reached a thickness of 100 feet in places in North 
America. A woolen mill 
built of it in Caledonia, 

New York, more than 100 
years ago is still in use. 

Several deiise varieties 
of travertine are formed by 
slow evaporation of hard 
water and show bands of 
different colors that are 
due to the presence of dif¬ 
ferent metallic oxides in 
the water depositing it, 
at different times. (Fig¬ 
ure 144.) 

The so-called Mexican onyx is the most attractive form 
of banded travertine and is much used for ornaments. (Fig¬ 
ure 145.) 

Another banded variety of travertine is the material of the 
stalactites and stalagmites shown in Figures 146A and 146B. 

Large deposits of travertine were formed along the shores 
of some former lakes, as for example, the deposits in Pyramid 
Lake, Nevada. (Figure 147.) 

Organic Sediments. — The sediments of this class con¬ 
sist of the shells or bones of animals or of the secretions of 
animals or of the decomposition products of plants. 

Limestone. — The material forming limestone, like that 
forming the chemical sediments, was dissolved from the soil 
and rocks by ground water. Streams carried the dissolved 
substances to lakes and to the sea where the dissolved lime¬ 
stone was absorbed by shellfish to form their shells, by coral 
organisms to form corals, and by the minute forms of marine 
life that live near the surface of the sea. 




318 


NEW PHYSIOGRAPHY 


The cast-off shells of these animals, when consolidated 
form fossiliferous limestone, the coral reefs became coraline 
limestone, and the hard parts of the surface life settled to 
the bottom of the ocean as calcareous ooze which became 
chalk when consolidated. 



Fig. 146 B . — Stalactites and Stalagmites, Carlsbad Cave, New Mexico 
Photo by Russell, U, S- G. S. 


Limestones formed near the continents are usually ren¬ 
dered impure by sand or mud brought by waves and shore 
currents. Pure limestone can only be formed in a region 
which does not receive such deposits. It may be formed 
in the deep sea, beyond the muds; or in shallow water, 
about coral islands; and in exceptional localities about con¬ 
tinents where sediment from the land is not being deposited. 

Properties. — (1) Limestone effervesces when tested with 
weak acid. 



THE BEDROCK 319 

(2) It is about the same hardness as calcite and may be 
scratched with a knife. 

(3) Since it is dissolved by water containing carbon dioxide 
or the weak acids resulting from the decay of humus, it is 
therefore one of the less durable rocks. 

(4) It is impervious to water. 



Fig. 147. — Travertine Pyramids, Pyramid Lake, Nevada 
Photo by Russell, U. S. G. S. 


Uses. — Limestone has more important uses than any 
other rock, and the value of the yearly output exceeds that 
of any other. When burned in a limekiln, it becomes quick¬ 
lime used in mortar and plaster. It is one of the principal 
ingredients of Portland cement. It is mixed with iron ore 
in the process of making pig iron. Ground limestone is 






320 


NEW PHYSIOGRAPHY 


spread on cultivated fields to correct acidity of the soil. 
About half the yearly output is used for road metal and 
for making concrete. 

Bituminous Coal. — That bituminous coal is of vegetable 
origin is shown by the plant remains found in the coal and 
in the clay beneath it and by the cellular structure shown 
by the microscope even in hard coal. 

It is estimated that a peat bed 300 feet thick would have 
been required to form a coal seam 20 feet thick. Such an 



Fig. 148. — Coquina, a Consolidated Bed of Modern 
Shells, St. Augustine, Florida 

From Ward’s Natural Science Establishment. Photo by Arey. 

accumulation would have required rank vegetation, a fact 
that is confirmed by remains of ferns and reeds as large as 
forest trees. 

There has been much discussion concerning the probable 
climate of the coal period. Some authorities think that it 
was cold and dry; others that it was warm and moist. But 
they agree that the peat accumulated in great marshes 
covering thousands of square miles and that the vegetation 
grew with its feet in the water. The rankness of the vegeta¬ 
tion also shows that it did not suffer for want of water. 




THE BEDROCK 


321 



The fossils found in the coal measures include tree ferns, 
cycads, palms, other greenhouse plants, as well as giant in¬ 
sects and alligatorlike reptiles that at the present time live 
only in warm climates. 

In our forests we have great accumulation of vegetable 
matter, but no coal is formed. This is because wood that 
decayed in air changes back to the substances from which it 


Fig. 149. — A Limestone Quarry 

was produced: namely, carbon dioxide and water, thus 
leaving no solid residue. 

When vegetable matter partially decays under water, it 
changes to marsh gas and coal. That peat and coal were 
formed under water is also indicated by the facts that peat is 
now forming under water and that remains of fish are oc¬ 
casionally found in the coal. We conclude that the coal 
regions were great marshes. Figure 150 shows some typical 
coal-forming plants growing under conditions that might 
build up the required layers of peat. (See also Figure 121.) 





322 NEW PHYSIOGRAPHY 

Properties. — Bituminous coal burns with much smoke 
and flame. This is due to the large amount of volatile mat¬ 
ter which it contains and which makes it valuable in the 
manufacture of artificial gas. It has a dull black color. It 
usually breaks along bedding planes parallel to the coal 
seam into pieces that are much more nearly rectangular 
than those of anthracite. 


Fig. 150. — Plants and Animals of the Coal Period 
From a sketch by Professor Williston. 

Metamorphism. — Some rocks are known to have been 
formed from sedimentary rocks, others from igneous rocks, 
and still others from other metamorphic rocks. 

Metamorphic Rock 

• 

The Nature of the Change. — Metamorphism usually 
results in an increase in the hardness and density of the rock, 
in its degree of crystallization, and often in the formation of 
an entirely new set of minerals. The change is accomplished 
without disintegration or removal of the former rock. 



THE BEDROCK 


323 


The extent of these changes depends upon the energy of 
the agents concerned and the time that the action lasts. As 
these two conditions have varied, they have produced very 
different degrees of metamorphism, from the slight altera¬ 
tion due to local causes to the profound changes due to enor¬ 
mous forces that affected regions with areas measured in 
thousands of square miles. 



Fig. 151. — Local Metamorphism Due to Hot Lava 

Metamorphosed rock is shaded: (1) quartzite formed from sandstone or siliceous con¬ 
glomerate, (2) slate formed from shale, (3) schist formed from shale, (4) marble formed 
from limestone. 

Local Metamorphism. — Bedrock is heated to a very 
high temperature when a stream of lava flows over it. The 
water within the stone is converted into steam, and the 
bedrock itself is slowly metamorphosed. After a time, how¬ 
ever, the lava cools, the steam escapes, and the changes in 
the rock remain slight. 

When lava wells up through a fissure forming a dike, or 
spreads out between the layers forming a sill (Figure 151), 
the lava cools more slowly because it is covered with a rock 










































324 


NEW PHYSIOGRAPHY 


blanket. The changes thus produced continue for a longer 
time, and are more pronounced. 

In the above diagram the metamorphosed rock is shaded. 
We have learned many facts from the numerous cases of 
local metamorphism found by geologists. Among them are 
the following: 

1. Quartzite may be formed from sandstone or from a siliceous 
conglomerate. 

2. Mica schist and slate may be formed from shale or mud. 

3. Marble may be formed from limestone. 

4. Metamorphic changes are greatest next to the hot lava and 
gradually become less marked at distances farther away from the 
lava. 

5. The rocks are metamorphosed for a greater distance on either 
side of the dike than they are above and below the sill, probably 
because steam can follow the bedding planes more readily than it 
can cross the layers. 

6. Clays and muds often form two distinct kinds of metamorphic 
rock. Next to the lava we sometimes find a mica schist, composed 
of flakes of mica with quartz and a few crystals of other minerals. 
This passes into a slate containing specks of black mica and other 
minerals. Beyond the slate the shale is unchanged. 

Table. — The relation between the deposits of the mantle rock 
and the sedimentary and metamorphic rocks which they form when 
consolidated and metamorphosed is shown in the following table: 


Mantle Rock 

Sedimentaky Rock 

Metamorphic Rock 

Clay. 

Shale. 

Slate — schist 

Sand. 

Sandstone. 

Quartzite 

Gravel. 

Conglomerate 

Quartzite, etc. 

Marl 1 

Shell Beds J 

Limestone. 

Marble 

Peat. 

Bituminous coal.... 

Anthracite coal (Graphite) 


Regional Metamorphism. — The large areas of metamor¬ 
phic rocks shown in Figure 136 extend over thousands of 
square miles. The rocks are more completely crystallized 
















THE BEDROCK 325 

than those altered locally and often possess a banded struc¬ 
ture like that of mica schist. 

It is probable that the metamorphic rocks of these regions 
were buried beneath miles of other rocks and therefore sub¬ 
jected to very high temperatures and great crushing and 
shearing forces. This would convert the rock into a pasty 
mass in which crystals would grow more easily in a direction 



Fig. 152. — Banded and Contorted Gneiss 
Hudson Bay. Photo by A. P. Low, Canadian Geological Survey. 


at right angles to the crushing force, thus developing a schis¬ 
tose structure, as the ability to split along uneven but nearly 
parallel surfaces is called. The same conditions would 
cause the wavelike lines seen so frequently in many meta¬ 
morphic rocks. (See Figure 152.) 

Agents of Metamorphism. — The principal agents of 
metamorphism are heat, moisture, and pressure. A large 
part of the heat comes from the interior of the earth, partly 
by conduction through the rocks but mostly through actual 




326 


NEW PHYSIOGRAPHY 


upward motion of molten rock beneath folded mountains 
and through dikes and fissures. 

Heat is also produced by the transformation of the energy 
of friction and compression due to lateral pressure. Some 
is probably also due to radioactivity. Some pressure comes 
from the weight of overlying rocks, but the most effective is 
the lateral pressure produced by the contraction of the earth. 
Very little of the moisture concerned in regional metamor¬ 
phism comes from the rainfall, because deep-seated rocks 
are crushed and crowded together so that there are no cracks 
through which it can flow. But many crystals and rocks 
contain water. 

The greatest effect is produced when these three agents 
act together. Heat acting alone can transform clay into 
hard and durable brick and pottery, and pressure can pro¬ 
duce the cleavage of slate and the characteristic structure in 
the schists; but neither heat nor pressure can produce, un¬ 
aided, the complete metamorphism found in mountainous 
regions. 

Great lateral pressure, such as that due to contraction 
of the earth, may produce sufficient heat to metamorphose 
rocks without the aid of igneous intrusions. The meta- 
morphic rocks in parts of New England seem to have been 
formed in this way. When these agents are acting jointly, 
heat greatly increases the solvent power and the chemical 
action of liquids and gasses; and pressure greatly increases 
the action of heat by lowering the melting point of rocks. 

In mountainous regions where there was great lateral 
pressure and where great masses of igneous rock welled 
up toward the surface, we find the rock most perfectly 
metamorphosed. 

Quartzite. — Quartzite is a metamorphosed conglomerate 
or sandstone. A siliceous sandstone free from impurities 
will form a white quartzite, but there are many outcrops of 
red or yellow quartzite that owe their color to impurities in 
the sandstone. 


THE BEDROCK 


327 



No new minerals are formed by the metamorphism of sand¬ 
stone. The grains of sand grow into small crowded crystals 
by the deposition of silica from solution. 


Fig. 153. — Slaty Cleavage and the Bedding Planes 
Photo by Keith, U. S. G. S 


The separate grains of sand may be seen with a lens in 
some specimens of quartzite; but the growth of the crystals 
has filled most of the spaces between them, making the 



328 


NEW PHYSIOGRAPHY 


quartzite less porous, slightly denser, and much harder to 
work into building stones than is sandstone. In many speci¬ 
mens of quartzite the luster and the shell-like fracture of the 
quartz crystal appear. 

Slate. — Slate is a metamorphosed shale. It is some¬ 
what harder and much more durable than shale, it has a 
characteristic cleavage that depends upon the direction of 
the pressure, and it is independent of the bedding planes. 
This enables us to split slate into thin, smooth slabs of 
uniform thickness. (See Figure 153.) 

The reader may have noticed the odor of wet clay when 
the slate blackboards at school were washed. 

There are many slate quarries along the border line be¬ 
tween New York and Vermont and others along the line of 
the Appalachian Mountains from New York to Georgia. 
Maine is the only other state in which slate is now quarried 
in any considerable quantity. 

Uses. — Slate is used for roofing, for school slates and 
blackboards; for billiard tables, floor tiles, steps and flag¬ 
ging; as a support for electric fixtures, and in making imita¬ 
tion marble. 

Marble. — Marble is a metamorphosed limestone. No 
new minerals are formed by the change. The material of 
the limestone has assumed the crystalline form. 

In its perfect form it consists of interlocking crystals of 
calcite; sometimes so small as to be invisible, but in other 
specimens as large as those of granite. The impurities of 
the limestone also are often crystallized. 

Pure limestone makes a white marble. Colored marbles 
are formed from impure limestones. Sometimes the colors 
are arranged in streaks producing beautiful effects when the 
stone is polished. 

When fully crystallized, the stratified structure and the 
fossils of the limestone have been destroyed, but the marble 
retains all properties of the limestone that depend upon its 
composition, such as its hardness, its solubility in water con- 


THE BEDROCK 


329 


tabling carbon dioxide, and its effervescence when treated 
with acid. 

In commerce the most important property of marble is 
its ability to take and retain a high polish; in fact this seems 
to be the only requirement, for some very slightly metamor¬ 
phosed but polishable limestones are sold as marble. 

Distribution. —■ Vermont furnishes more marble than any 
other state in this country, supplying both white and varie¬ 
gated varieties. Some marble is quarried at Gouverneur, New 
York; also in California, Colorado, Maryland, Georgia, and 
Tennessee. 

Most of the highly colored marbles used for interior dec¬ 
oration are imported. France, Belgium, Greece, and Africa 
each supply large quantities. 

Uses. — Marble is used in masonry, wainscoting, paneling, 
and flooring. 

Quarry refuse is used for building roads, in making artifi¬ 
cial marble and Portland cement, and in the preparation of 
carbon dioxide. 

Mica Schist. — When mica schist is found under the con¬ 
ditions shown in Figure 151, we know that it was formed 
from shale; but there are several igneous rocks with the 
same chemical composition as shale, and it is possible that 
metamorphism may have produced mica schist from them. 

Mica schist is composed chiefly of crystals of quartz and 
mica, the mica being greatly in excess. The flakes of mica 
are nearly parallel to each other which causes the schistose 
structure before mentioned. 

Gneiss. — The common variety of gneiss is composed of 
crystals of quartz, feldspar, and mica arranged in parallel 
lines. Its appearance suggests that it may have been formed 
from granite by heat and pressure. 

Both mica schist and gneiss are widely distributed in 
North America. They are common in the central portions 
of many mountain ranges. 

Anthracite Coal. — Beds of bituminous coal which have 


330 


NEW PHYSIOGRAPHY 


been baked into a natural coke are to be found in Virginia 
and North Carolina, and beds that have been changed to 
anthracite are found in Colorado. 

Thin sections of anthracite seen under the microscope 
show cellular structures indicating its vegetable origin. For 
these reasons we suppose that our large fields of anthracite 
were formed from bituminous coal by metamorphism. It 
differs from bituminous coal in that it contains very little 
volatile matter and burns without the large amount of smoke 



Fig. 154. —■ Coal Beds of the United States 


and flame characteristic of bituminous coal. It is a dense, 
hard, lustrous substance which does not break along bedding 
planes as does soft coal, but has an imperfect shell-like 
fracture. 

Distribution. — Figure 154 shows the known coal fields 
of the United States in black. How many of our 48 states 
have coal fields? In 1925 Pennsylvania produced 34.3 per 
cent of all the coal mined in this country; West Virginia 
was next, producing 21 per cent; and Illinois third with 
11.8 per cent; Kentucky fourth with 9 per cent. These 










THE BEDROCK 


331 


four states produced more than three-quarters of all the 
coal mined. Alabama, Indiana, and Ohio each mined 
about 5 per cent. 

Anthracite is found only in regions of disturbed and folded 
strata. 

About 300,000 square miles of land in the United States 
are underlaid with coalbeds. Not all of this is workable, 
because of its impurity or the thinness of the layers, and 
the area at present producing coal is but a small fraction 
of the total. Very large areas in Alaska are also coal lands, 
but they are at present undeveloped. All varieties of coal 
are found in the United States, from the graphitic anthracite 
of Rhode Island, which burns with great difficulty, to the 
lignite of Texas, which retains much of its woody structure. 

The amount of coal mined in the United States exceeded 
that of any other nation, reaching the total of more than 
500,000,000 tons in 1925. 

You will note that there are large coal fields on the west¬ 
ern slope of the Appalachian Mountains, on both sides of the 
Mississippi River, extending from the latitude of Iowa to 
the Gulf, and on the eastern slope of the Rocky Mountains. 
They are well distributed over the whole valley of the Mis¬ 
sissippi and its tributaries. Our map shows only the known 
beds. There may be others buried beneath deep covers, and 
doubtless much coal has been worn away during the millions 
of years since the coalbeds were formed. 

Petroleum and Natural Gas. — For the past 50 years 
we have been taking from the bedrock great quantities of 
petroleum and natural gas which have accumulated during 
the ages. 

The value of these products secured during 1924 was 
about $1,500,000,000. which exceeds the value of all the 
metallic ores mined in that year. 

Gasoline, kerosene, and lubricating oils are obtained from 
petroleum and furnish much of our motive power, our heat, 
and our light. 


332 


NEW PHYSIOGRAPHY 


The distribution of oil and natural gas in the United 
States is shown in Figure 156. Large fields have been dis¬ 
covered in Louisiana, Texas, Oklahoma, and California, but 
the demand for the petroleum products is so great that the 
present-known fields will probably be exhausted before long 
so that we shall then have to find substitutes for gasoline. 



Fig. 155.— A Group of Oil Wells 


Valuable Minerals 

In addition to the rock-making minerals, some of which 
are used quite extensively in the arts as already shown, 
there are a number that are included because of their eco¬ 
nomic importance. 

The Gems. — Some of the gems found in the bedrock 
have yielded enormous sums of money. Three large dia¬ 
mond mines of South Africa yielded a total of 40,000,000 
carats of diamonds, more than eight tons, in the 17 years 
preceding 1889. 

The best oriental rubies and sapphires come from Upper 
Burma, north of Mandalay, where they are found in crystal¬ 
line rocks. They are aluminum oxides colored by varying 
amounts of other minerals. 







THE BEDROCK 


333 


The annual yield is not nearly equal to that of the dia¬ 
mond, but the best specimens are more valuable than a 
diamond of equal weight. 

Native Metals. — Several metals which occur in their pure 
state in the rocks are called native metals. They are occasion¬ 
ally found in large masses, but more frequently are scattered 
through the rock in small particles. The principal native 
metals are gold, silver, platinum, mercury, and copper. 

Gold and Silver. — The principal uses of gold and silver 
are as coin, bullion, and jewelry. They are also used quite 
extensively in the arts. There is some loss of the metals due 
to their uses in electroplating and in manufacturing photo- 



Fig. 156. —• Distribution' of Petroleum and Natural Gas Fields 
in the United States 


graphic supplies. The world’s supply of gold and silver is 
undoubtedly increasing. The leading gold-producing states, 
California, Colorado, South Dakota, and Nevada, in 1925 
yielded about $30,000,000 of gold. Nevada, Montana, 
Utah, Colorado, and Idaho are the leading silver-producing 
states. 
































334 


NEW PHYSIOGRAPHY 


Platinum. — Platinum is much more valuable than gold. 
No comjnon metal is as well adapted for use in electrical 
and chemical apparatus. Most of the platinum of commerce 
comes from the Ural Mountains, but small quantities are 
found in California, Australia, and Brazil. 

Copper. — The Lake Superior copper region, near Kewee¬ 
naw Point, Michigan, is the most important deposit of 
native copper in the world. This metal also occurs in Ari¬ 
zona, Montana, and in small quantities in several other 
states. 

Many fragments, torn from the outcrops of copper, are 
found in the glacial deposits of glaciated regions, sometimes 
at great distances from the outcrop. “ Drift copper ” of 
this sort probably supplied prehistoric men with their bronze 
implements. 

The Ores. Copper Ore. — Thousands of tons of copper 
are obtained by chemical processes, from compounds of cop¬ 
per with sulphur or with carbon dioxide. These are the chief 
ores of copper. An ore is a substance from which a metal may 
be extracted, but no substance is considered an ore unless it 
contains the metal in workable quantities. Copper is used 
chiefly for the manufacture of wire, sheet copper, and brass. 

Although there is little necessary loss of copper and brass, 
all scrap copper and brass should be saved; for neither 
of the states mentioned above as supplying copper in quan¬ 
tity, has enough ore to last more than a few years longer at 
the present rate of mining. The value of the yearly output 
of copper is double that of gold and silver together. 

Iron. — At present this is our most important metal. 
The value of the yearly output of our iron mines exceeds 
that of the mines of all the other metals put together. 

The great value of iron in the industries is due to three 
properties: (1) Its ore is easily reduced to metallic form. 
(2) It may be given a desired form by hammering when red 
hot, or by pouring it into a mold when melted. (3) It may be 
converted into steel having great hardness, or great tough- 


THE BEDROCK 


335 


ness, or great tenacity, as may be desired. The chief dis¬ 
advantage of iron is its tendency to rust when exposed to 
air and moisture. Considerable progress has been made in 
overcoming this property. In recent years the Lake Supe¬ 
rior region has supplied about 80 per cent of the iron ore 
mined in this country. 

There are three important ores of iron; 

Limonite is a compound of iron, oxygen, and water. It is similar 
to the red rust that forms on iron at ordinary temperatures. 

Hematite is a compound of iron and oxygen without water. It 
has a black or red color, contains 70 per cent of iron, and is more 
generally used than the other ores. It occurs in beds in sedimen¬ 
tary and metamorphic rocks, though sometimes it is found in 
small crystals distributed through igneous rocks. 

Magnetite is a compound of iron and oxygen corresponding to 
the rust formed on iron at high temperatures. Its composition is the 
same as that of the scale found about a blacksmith’s anvil. It 
occurs as small crystals diffused through igneous rocks and also in 
veins which are mostly confined to metamorphic rocks. As its 
name indicates it is a natural magnet. 

Since iron may be used over and over again, there is little danger 
that future generations will lack iron, if all scrap iron is saved. Coal 
can be used but once, and the time is surely coming when it will be 
found only in museums. 

Aluminum. — This is the most abundant metal found in 
the lithosphere. It does not occur free in nature, but its 
compounds are very common. Every clay bed contains a 
large percentage of it, but we have as yet no cheap process 
of extracting the metal from clay. 

The principal ore of aluminum is an oxide, called bauxite 
(first found at Baux, in France). 

It is a white metal that does not tarnish easily. It is very 
light, yet rigid. It is tough, malleable, ductile, and it is a 
better conductor of electricity than copper of the same 
weight. 

It is used in cooking utensils, in paints, and in all ma- 


336 


NEW PHYSIOGRAPHY 


chinery where lightness and strength are important. There 
are several deposits of bauxite in the United States, those 
in Georgia, Alabama, and Arkansas yielding much ore. 

Lead. — The principal ore of lead is galena , a sulphide 
having a metallic luster and a highly perfect cubic cleavage. 

The occurrence of lead in the Mississippi Valley was known 
to the Indians before the white man appeared. Until the 
middle of the last century, Wisconsin and Illinois supplied 
most of the lead ore used; but at the present time Missouri, 
Idaho, and Utah each exceed their yield. 

About 30 per cent of the lead mined in this country is used 
in making white lead for painting. This can be used but 
once. Lead is also used in storage batteries, in plumbing, and 
in making solder. Unless the use of white lead is very much 
diminished, the metal is doomed to disappear in a compara¬ 
tively few years. 

Zinc. — The principal ore of zinc is sphalerite, a sulphide 
called blackjack by the miners. It occurs in several states, 
Missouri, Kansas, and Illinois leading. 

The principal use of zinc is in galvanizing iron to prevent 
rusting. It is also used in paint and in dry batteries. For 
these purposes zinc is used only once. The portions used 
for sheet zinc and in making brass, however, are not lost. 

Mineral Fertilizers. — The most important mineral fer¬ 
tilizers found in this country are the rock phosphates of 
Tennessee, South Carolina, and Florida. These rocks owe 
their value chiefly to the great number of bones of mastodons 
and other animals of which they are largely composed. They 
are efficient fertilizers of lands needing phosphorus. Sodium 
nitrate (Chile saltpeter) occurs in beds several feet in thick¬ 
ness in the northern part of Chile, and also in Humboldt 
County, Nevada, and in California. It supplies about 15 
per cent of its weight of nitrogen. One of the most valuable 
mineral fertilizers used to supply potash is saltpeter. Salt¬ 
peter is formed abundantly in certain soils in Spain, Egypt, 
and Persia, and also in considerable quantities in the soil of 


THE BEDROCK 


337 


many caves in the Mississippi Valley. It yields about 45 
per cent of its weight of potash, and 13 or 14 per cent of 
nitrogen. 

Graphite. — Graphite, commonly called black lead , is 
pure carbon, being identical in chemical composition with the 
diamond. It is a soft dark gray mineral with a greasy feel 
and a metallic luster, and derives its name from its property 
of leaving a mark on substances. 

Occurrence. — Extensive graphite mines are located at 
Ticonderoga, New York, and in several western states. A 
superior quality of graphite is found on the island of Ceylon 
and also in Siberia. 

Uses. — Graphite is used in making crucibles in which to 
fuse metals, in lead pencils, stove polish, electric light car¬ 
bons, and lubricants. 

Sulphur. — Sulphur is a soft yellow mineral that burns 
with a blue flame and a characteristic odor. It is some¬ 
times found in transparent yellow crystals with a glassy 
luster. 

Occurrence. — Sulphur crystals are found in deposits 
formed by volcanoes. Large quantities are obtained from 
Vesuvius and other active volcanoes, and it is recovered 
from the deposits of extinct volcanoes in Nevada, Utah, and 
California. 

Sulphur also occurs in the cap rock of the oil domes of 
Louisiana and Texas, in some cases forming as much as 
50 per cent of the rock. From this rock it is recovered by 
forcing hot water under great pressure at a temperature of 
350° F. down to the rock through a large pipe. The water 
melts the sulphur, and the mixture is raised to the surface 
by the air-lift method of pumping. 

This process is yielding about 2,000,000 long tons a year, 
which is about 500,000 long tons more than the annual con¬ 
sumption in the United States. 

Uses. — Sulphur is extensively used in the manufacture of 
sulphuric acid, the most important chemical agent that we 


338 


NEW PHYSIOGRAPHY 


have. It is also used in the manufacture of fertilizers, fire¬ 
works, explosives, paper, wood pulp, and rubber. It is also 
burned for bleaching and disinfecting purposes. 

Mineral Resources of the United States. — The following 
figures are based on the preliminary summary of our mineral 
resources for 1925, published by the United States Geologi¬ 
cal Survey. They give in round numbers an estimate of the 
value of the materials extracted from the lithosphere in 1925, 
which amounts to about $15,000,000 for every day of the 
year. 


Summary for 1925 
Metallic 


$ 739,000,000. 

238,000,000. 

50,000,000. 

46,000,000. 

307,000,000. 

$1,380,000,000. 

Nonmetallic 

$ 328,000,000. 

1,046,000,000. 

51,000,000. 

1,270,000,000. 

254,000,000. 

286,000,000. 

171,000,000. 

4,000,000. 

12,000,000. 

1 , 200 , 000 . 

106,000,000. 

43,000,000. 

12,000,000. 

732,000,000 . 

$4,316,200,000. 
$5,696,200,000. 

Erosion. — We use the general term u erosion” to desig¬ 
nate the act of wearing away the land. The principal proc¬ 
esses are stream erosion, glacial erosion, wave erosion, and 
wind erosion. Each of these processes is discussed in other 
chapters. 

Erosion is materially aided by weathering, which disin- 


Anthracite coal. 

Bituminous coal. 

Coke. . .. 

Petroleum, crude, at wells. 

Natural gas (1924). 

Cement. 

Stone. 

Abrasives. 

Fertilizers, phosphate rock 

Fertilizers, potash. 

Sand and gravel. 

Lime. 

Slate . 

Unspecified. 


Pig Iron. .. 

Copper. 

Gold. 

Silver. 

All other metals 
























THE BEDROCK 


339 


tegrates rock and supplies the loosened fragments to the 
agents of erosion. 

Weathering loosens rock fragments but does not trans¬ 
port them. Each of the agents of erosion loosens rock frag¬ 
ments and also removes them, depositing them elsewhere. 
On the physical appearance of the land both erosion and 
deposition leave their marks, and the agent that changed or 
is changing this physical appearance may be readily detected. 

The vastness of the work performed by the processes of 
erosion will be appreciated if we consider the products of 
their work; i.e ., the deposits of rock waste derived from 
older rocks. 

1. The mantle rock, covering most of the known land and some¬ 
times reaching a thickness of hundreds of feet, is made up of frag¬ 
ments of older rocks that were eroded. 

2. The sedimentary rocks that underlie a large part of the mantle 
rock and that reach a thickness in places of many thousand feet 
are composed of matter eroded from former land. 

3. Metamorphic rocks that were once sedimentary are also com¬ 
posed of eroded material. 

4. The corals, oozes, and muds of the ocean floor, except in the 
deeps, are made of eroded material. 

5. The millions of tons of mineral matter held in solution in sea 
water were carried from the land to the sea by rivers. 

To produce this great mass, a quantity of solid rock much 
greater than the sum of the masses of the mantle rock, the 
sedimentary rock, and the material on the ocean floor must 
have been worn away; for sea water contains millions of 
tons of matter dissolved from both bedrock and mantle rock, 
and portions of the bedrock have been deposited and worn 
away again more than once. 

If all this material came from the land at present known, it 
is evident that the total is equivalent to the removal of 
thousands of feet of bedrock from all the land of the earth. 
This great quantity of material has not been removed with 


340 


NEW PHYSIOGRAPHY 


equal rapidity in all parts. The rocks have been worn away 
more rapidly where they were weak or where the agents were 
particularly active, making the land uneven, developing val- 



Fig. 157. — Effects of Erosion on a Hard, Sandy Clay 
Foot of Scott’s Bluff, Nebraska. 

leys, canyons, gorges, and fiords; and leaving mountain peaks 
and ridges, or mesas and buttes. 


QUESTIONS 

1. Mention six gems that belong to the quartz group of minerals. 

2. State two properties common to both quartz and feldspar. 
State two properties, either one of which would distinguish quartz 
from feldspar. 

3. Find five illustrations in this book showing stratified rock, 
and three showing unstratified. 

4. State four properties in which calcite differs from feldspar. 

5. State two points of resemblance and two of difference between 
sandstone and shale. 

6. Which is the more common surface rock in the United States, 
sedimentary, igneous or metamorphic? Which least common? 

7. What states yield coal? See Figure 154. 



THE BEDROCK 341 

8. How could you determine whether a given specimen was 
sandstone or limestone, if you had no acid? 

9. Why is rock forming near the shore of some coral islands a 
pure limestone? 

10. How may slate be distinguished from shale? 

11. Coarse-grained granite is now on the surface in New Eng¬ 
land; what does this prove concerning the elevation of the surface 
in these localities at the time when the granite was forming? 


CHAPTER XVIII 


“ STORIES IN STONES” 

The Development of Life. — As we walk along the sea¬ 
shore, we note that the beach is strewn with shells and 
seaweed and that many living animals make their homes in 
the sands. Occasionally we see a fish that has been washed 
ashore or find specimens from the plant or animal life of the 
land, such as leaves blown by the wind or bones brought by 



Fig. 158. — Fossil Fish from Wyoming 
Photo by Arey. 


some predatory animal. This litter on the beach is soon 
buried in the sand brought in by the waves. 

If such a beach should be consolidated, the sandstone 
formed would preserve the hard parts, and often the impres¬ 
sion of the softer parts of the animals and plants buried in 
it. These remains of animals and plants and the impressions 
that they form in the rock are called fossils. 

In Figure 158 the bones of the fish are distinctly seen; and 
the whole animal is shown in a manner that resembles an 
X-ray picture of a modern fish, although this fish had proba¬ 
bly been in his rock tomb millions of years. 

342 



STORIES IN STONE” 


343 


There is a great series of sedimentary rocks containing 
just such a record of the forms of plant and animal life of 
the past; and since each layer is older than all of those de¬ 
posited above it, we are able to determine the order in which 
the various types of life occupied the earth’s surface. The 
various layers may be likened to the leaves of a book, each 
one of which bears a record of the kinds of life which dwelt 
upon the earth at a given time. 

The thousands of leaves of nature’s history have been 
divided into several parts — call them volumes if you like — 
each of which holds the record of the life of a certain era 
which was terminated by considerable changes in the life of 
the world and which was also marked by important physical 
changes in the land. 

Below the sedimentary rock is a layer of metamorphic 
rock most of which was originally sedimentary. Recent 
research has shown that these rocks — known as the pro - 
terozoic series — also contain remains of both plant and 
animal life. 

We find still lower a series of more perfectly metamor¬ 
phosed rocks, most of which were originally igneous, but in 
some of those of sedimentary origin the remains of micro¬ 
scopic plants have been found. These rocks, known as the 
archeozoic series, contain the earliest record of plant life 
that has been discovered. 

If we represent all the known rocks as in a great pile, with 
the oldest at the bottom, and divide the mass by horizontal 
lines into spaces to represent the eras, with a width propor¬ 
tional to the thickness of the rocks belonging to each era, 
we shall have a handy reference table that will help in form¬ 
ing a mental picture of the events of this history of the earth. 
Evidences of the existence of plant or animal life are so 
seldom found in archeozoic rocks that for many years geolo¬ 
gists believed that there was no living thing in existence 
during that era. 

The rocks are so completely metamorphosed that, even 


344 


NEW PHYSIOGRAPHY 


The Geological Column 


Eras 

Ages 

Remarks 

Dominating Rocks 

Present 

Age of man 

Modern life 

Mantle rock 

Cenozoic 

Age of mammals 

Flowers 

Sedimentary 


Interval, duration unknown Rise of archaic mammals 




Toothed birds and 


Mesozoic 

Reptiles 

“dragons” 

Sedimentary 


Flying reptiles 

--—. 


Interval, duration unknown Extinction of ancient life 



Amphibians 


Sedimentary 

Paleozoic 

Fishes 

Lung fish 
sharks 

Sedimentary 

Invertebrates 

Scorpions 

Cephalopods 

Trilobites 

Sedimentary 




Interval of lost rocks Continents again base-leveled 




Protozoa 

Sedimentary, only 

Proterozoic 

Primitive inver¬ 

“The great iron age” 

slightly meta¬ 


tebrates 

Marine plants 

morphosed 


Interval Continents base-leveled 




“Sea weeds” 

Igneous much met¬ 

Archeozoic 

Larval life 

Algae 

amorphosed 


if life had been abundant, few of the fossils in the original 
rocks would have been preserved. Careful search, however, 
has been rewarded by undoubted proof that certain low 
forms of living organisms existed. 

































































“STORIES IN STONE 


345 


The fossil seaweed shown in Figure 159 proves that plants 
were in existence very early in the archeozoic era. The fossil 
was found in a pebble from an archeozoic conglomerate, and 
therefore was living long before the pebble took its place in 
the gravel bed that was consolidated to form the conglom¬ 
erate. Figure 159 is a microphotograph of the oldest form 
of life known. 

Archeozoic rocks contain much graphite. It is said that 



Fig. 159. — Blue-Green Alga from Archeozoic Rocks 
Courtesy of Professor J. W. Gruner. 

there is as much carbon in the form of graphite in this group 
of rocks as there is in the form of coal in the Pennsylvania 
coal measures, and also that this indicates that plant life 
was very abundant. 

The lower part of the proterozoic era contains coral-like 
masses secreted by a low order of plants, in great abundance 
and widely distributed. There are several varieties of them, 
one of which is shown in Figure 160. 





346 


NEW PHYSIOGRAPHY 


The upper part of the proterozoic series of rocks contains 
unquestionable remains of protozoa, a term applied to the 
first animal life. 

The Paleozoic Era. — The lowest rocks of this era contain 
fossils of the animal and plant life of the era in great abund¬ 
ance. More than 
twelve hundred species 
of animals have been 
identified in North 
America alone, repre¬ 
senting all of the im- 
portant types of 
marine invertebrates. 

, Figures 161 and 162 
show fossils from these 
rocks. All of them 
were complex orga¬ 
nisms; and when 
compared with the 
microscopic seaweeds 
that were the domi¬ 
nant living forms of the archeozoic, they show an advance 
that must have required an interval of millions of years to 
accomplish. 

The trilobites, Figure 161, were the most abundant and the 
highest form of early paleozoic life. They had an external 
bony covering like that of the lobster and the horseshoe 
crab. Some of the early ones appear to have been blind, or 
at least they had no eyes on their upper side; others had 
many eyes that could not be moved, and they therefore 
needed separate eyes for each direction in which they wished 
to see. One species was able to look in 98 different directions 
at the same time. 

In the rocks of the second quarter of paleozoic time we 
find an increase in the number of species of trilobites. These 
had a higher degree of development as illustrated by the 




STORIES IN STONE 


347 


compound eyes of the specimen shown in Figure 1635, each of 
which has a range of vision of more than 200°. Think of an 
animal that could see behind 
him as well as in front of him, 
and on all sides at the same 
time without turning his head! 

It would be difficult to sur¬ 
prise such an animal, and yet 
during this period the trilobite 
lost his proud position as Lord 
of Creation to a smarter and 
more active animal of the 
nautilus family, the cephalo- 
pod (see Figure 164). The 
reign of the cephalopod was 
of short duration, relatively; 
for before the close of the age 
of invertebrates the first fish 
was living. The fish has a 
backbone and is called a verte¬ 
brate. . It was more active and in many respects a higher 
type of animal life than any of the invertebrates then living. 

Figures 163, 164, and 165 are photographs of fossils from 
the upper layers of the rocks belonging to the age of inverte¬ 
brates. In these layers the remains of sev¬ 
eral forms of invertebrates are much more 
abundant than those of fishes or of scorpions. 

The rocks of the closing years of the age of 
invertebrates give us the scorpions, the old¬ 
est known air-breathing animals, the first 
type that could live on the land. These 

Fig. 162. _ a pai.eo- rocks also give the first satisfactory evidence 

zoic brachiopod 0 f the existence of land plants. 

or Lamp Shell ^ 

That reef-building corals were at work at 
this time is shown by the long coral reefs that have been 
traced in these rocks across several states. 






Fig. 161.—An Early Paleozoic 
Trilobite 





348 


NEW PHYSIOGRAPHY 


The Age of Fishes. — At the beginning of this age we find 
evidence that the land was clothed with vegetation for the 
first time and that several species of air-breathing animals 
spread over the land. 

In the sea there was a great increase in the number of 
fishes and a decrease in the numbers of trilobites and cepha- 
lopods. 

Figure 166 is an imaginary portion of the bottom of the 
sea as inhabited by invertebrates such as lived in the early 

part of the age of fishes. In 
the center is a very orna¬ 
mental cephalopod with its 
octopuslike arms about a 
trilobite on which it is 
feeding. 

Three other trilobites and 
a number of other quaint 
forms of animal life are 
shown; among them, the 
crinoid at the extreme left, 
which was for a long time 
believed to be a plant. The 
top of it resembles a flower 
in some respects; and it is 
attached by a slender 
“ stem ” to a branching 
mass, suggesting roots, 
which is buried in the sand. 
It was formerly called a 
stone lily , but is now known 
to be an animal. 

The fishes in Figures 167 and 168 were also found in the 
lower rocks of this age. The lung fish is so called because its 
air bladder had become a sort of primitive lung that enabled 
it to live in temporary waters. 

Toward the close of the age of fishes the fossils show that 


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Fig. 163A. — A Late Paleozoic 
Trilobite 





STORIES IN STONE” 


349 


trilobites had almost disappeared but that the crinoids, at 
left in Figure 166, were more abundant than they have 
ever been since. Conditions at this time seem not to have 



Fig. 163 B . — Eye op the Same Fossil, Greatly Enlarged 
Photo by American Museum of Natural History. 


been favorable to vegetation, which was “ impoverished, re¬ 
stricted, and stunted.’’ 

The Age of Amphibians. — This age is characterized by 
the great abundance of land plants that grew luxuriantly in 
great swamps covering most of the interior of the United 



350 


NEW PHYSIOGRAPHY 


States. Several of the more common trees of the coal meas¬ 
ures are shown in Figure 150 as restored by Professor 
Williston. 

Amphibians are animals such as the frog, which begin life in 
the water and later develop lungs and live on land. Two 

ancient amphibians are 
seen in Figure 150 as is 
also one of the great 
insects, of the dragon-fly 
type, which measured 29 
inches across the wings, 
the largest ever known. 
Eight hundred kinds of 
cockroaches have been 
found in the rocks of this 
age. 

As the close of the 
paleozoic era approached, 
three events occurred 
that tended to diminish, 
or to change the charac¬ 
ter of, the life of the con¬ 
tinents : 

1. The drainage of the 
coal swamps obliged the 
plants that had been growing with their feet in the water to 
adapt themselves to dry-land conditions, and this great 
change undoubtedly exterminated all of the invertebrates 
that lived in the swamp waters. When the area that had 
been covered by a shallow sea extending from California to 
Texas and from Montana to Mexico became dry land, there 
was further extinction of the life of the sea. The arid condi¬ 
tions so prevalent at this time destroyed animal and plant 
life in all the seas that were depositing salt and perhaps in 
those that were depositing gypsum. 

2. The elevation of the Appalachian Mountains and the 



Fig. 164. — Paleozoic Cephalopods 



STORIES IN STONE 


351 


ancestral Rockies affected the climate of a considerable area, 
producing conditions that conceivably may have been inimi¬ 
cal to the slow-moving portion of the land animals. 

3. Finally the glacial cold of the period undoubtedly ex¬ 
terminated many animals. 

The Mesozoic Era. — Two of the dominant forms of paleo¬ 
zoic animal life, the trilobite and the sea scorpion, had dis¬ 
appeared before the be¬ 
ginning of the mesozoic 
era; and the crinoids, 
brachiopods, and mol- 
lusks had begun to as¬ 
sume a more modern form. 

The mesozoic era is the 
age of reptiles. 

“ They filled all of the 
roles now taken by birds 
and mammals; they 
covered the land with 
gigantic herbivorous and 
carnivorous * forms, they 
swarmed in the sea, and, 
as literal dragons, they 
dominated the air” 

(Scott). 

Figure 169 shows the 
largest known flesh-eat¬ 
ing dinosaur, the “ tyrant 
king of the saurians,” 

47 feet in length and standing 18| feet high. The weight 
of the dinosaur was so great that, in spite of the great 
strength of his hind legs, he was obliged to use his tail as a 
third support. His forelegs were not used for walking. 

Figure 170 represents a veritable “ dragon of the air.” Its 
wings spread 16 feet. This is the earliest known bird and 
one of the forms that lived when reptiles were learning to fly. 



Fig. 165. —• A Fossil Scorpion 
From a drawing by Dr. Peach. 







352 


NEW PHYSIOGRAPHY 




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STORIES IN STONE 


353 


The withdrawal of the great seas from the interior of the 
continent and the draining of the swamps that bordered 
them, together with the increasing severity of the winters 
caused by the mountains of the Laramide Revolution, brought 



Fig. 167. — An Early Shark 
British Museum. 


about the dramatic extinction of the great dinosaurs and the 
toothed birds that occurred at the close of the era. 

Cenozoic Era or the 4 ‘Age of Mammals.” — The modern 
mammal is the highest type of animal life known, and its 
greater intelligence and activity soon gave it a dominating 
position. The earliest mammals had certain reptilian charac¬ 
teristics. They disappeared suddenly, leaving the modern 
mammals in full possession of the land and with certain mam- 



Fig. 168. — Lung Fish 
British Museum. 


mals like the seal, sea lion, sea cow, and the whale in posses¬ 
sion of the water. The fossil fish in Figure 158, though it 
came from cenozoic rocks, is remarkably like the fishes of 
our own age. 

It was not known that a woolly member of the elephant 
family ever existed, even though fairly good drawings of 










354 


NEW PHYSIOGRAPHY 


them, executed by prehistoric men on ivory, were well known. 
The body of one was found at the mouth of the Lena River 
some years ago, and the hide and the skeleton were brought 



Fig. 169. — The Tyrant King of the Dinosaurs 
Courtesy of the American Museum of Natural History. 



Courtesy of the American Museum of Natural History. 

to this country and mounted, the skeleton in Chicago and 
the hide in New Orleans. 

Summary. — The earliest known plant is a microscopic 
seaweed found in the archeozoic rocks. At the beginning of 
the paleozoic era, certainly, and probably much earlier than 
that, the oceans teemed with plant life in sufficient quantities 














“STORIES IN STONE 


355 


to feed millions of invertebrates. Early in paleozoic time 
plant life spread over the land, and by the beginning of the 
age of fishes forests 
appeared. Then came 
the coal plants with 
their luxuriant vegeta¬ 
tion. Since that time 
the modern species of 
plant life have been 
gradually introduced. 

The oldest known ani¬ 
mals were microscopic 
protozoa such as are 
now found in the calcareous oozes of the ocean. With the 
beginning of the paleozoic era we find well-developed marine 



Fig. 171. 


Head of the Earliest Known Bird 
Restoration by Heilmann. 



Fig. 172. — The Eohippus, the Four-Toed Horse 
Restoration by C. R. Knight. Courtesy of the American Museum of Natural History. 


fauna consisting of more than twelve hundred species of ani¬ 
mals, all invertebrates, with the trilobite as king. 

A little later a higher type of animal, the fish, appeared 






356 


NEW PHYSIOGRAPHY 


and succeeded to the throne of the animal kingdom. Gradu¬ 
ally some of the fish developed lungs and the ability to live 
out of water. Then the amphibian appeared. Next the rep¬ 
tile began to flourish in the rocks of the coal measures. 

The layers of rocks of the age of reptiles are filled with re¬ 
mains of giant reptiles that ruled the land, the sea, and the 



Fig. 173. — The Woolly Mammoth 
Courtesy of the American Museum of Natural History. 


air. The saurians were indeed the “ Lords of Creation ” for 
millions of years; but mammals appeared, and with their 
appearance the decline of the reptile began. 

The greater brain capacity of the mammal and their 
greater activity entitled them to the dominating position, 
and we are not surprised that the great reptiles of the meso- 
zoic era were succeeded in the cenozoic era by great mammals, 
which ruled the land and sea while birds dominated the air. 

These are the facts concerning the history of plants and 




STORIES IN STONE 


357 



Fig. 174. _Restoration Drawing of the Eoanthropous, One of the Earliest Men 

By A. Forestier. Copyright. Courtesy of the London Illustrated News. 


animals. They began as microscopic forms, of few species 
with simple structure and little intelligence. They have de¬ 
veloped into the flora and fauna of today with man as the 
“ Lord of Creation.” 

The great changes in the animals were brought about 




358 


NEW PHYSIOGRAPHY 


slowly by many slight changes. The chief line of develop¬ 
ment, however, was not increasing strength; neither was it 
increasing size. Through it all we see that the important 
change was the development of higher and higher mentality. 

Will man be succeeded in the future by some new form of 
animal of higher intelligence, just as his predecessors have 
been? 

Chronology. — In an effort to estimate the number of years 
that each era lasted, geologists have studied the amount of 
sediment deposited, the quantity of rock eroded, the building 
and base-leveling of mountains, and other facts bearing on 
the subject. These scientists think that the following com¬ 
parison gives the most accurate idea of the enormous length 
of geological time: 

“ Let a year be represented by a foot. The average life 
of a human being is then represented by the breadth of a 
dwelling house, and human history is limited to approxi¬ 
mately a mile; but the duration of geologic time is com¬ 
parable to the circumference of the globe.” 


QUESTIONS 

1. Account for the occurrence of fossils in sedimentary rocks. 
Why are they more abundant in limestone than in sandstone? 

2. How do the sedimentary rocks show us the order in which the 
different forms of life appeared? 

3. Give reasons for separating the rock record into eras. 

4. What great change occurred at the beginning of the paleo¬ 
zoic era. 

5. How does the life of the paleozoic compare with that of 
the proterozoic? 


CHAPTER XIX 

THE GROUND WATER 

Disposal of the Rainfall. — The rain that falls on the 
ground is disposed of in one or more of the following ways: 
(1) It may evaporate and rise into the air. (2) It may re¬ 
main on the surface, flowing toward the sea as streams or 
collecting in hollows as lakes and swamps. This portion of 
the rainfall is known as the run-off. (3) It may sink into 
the ground. This portion of the rainfall is known as the 
ground water. 

The percentage of the rainfall disposed of in each of these 
three ways depends upon the temperature and dryness of 
the air, which control the rate of evaporation; upon the 
slope of the surface of the land, which determines the time 
that the run-off remains on the land; and upon the per¬ 
meability of the soil, which affects the percentage that be¬ 
comes ground water. 

In hot, arid regions most of the rainfall evaporates. The 
rain that falls on steep hillsides, on frozen soil, on impervious 
rock, or on the roofs and paved streets of a city quickly joins 
the streams as a part of the run-off. 

Most of the rain that falls on well-tilled soil, that is loose 
and porous, becomes ground water. 

The highest percentage of the rainfall becomes ground 
water in cool, level regions with porous soil; and the lowest 
percentage in regions of steeply sloping, impervious rock or 
frozen soil. 

Sources of the Local Ground Water. — The ground water 
below a given locality may be derived chiefly from local rain¬ 
fall, but it may also flow from higher land, if the subsoil is 
porous; or it may come in part from fissure springs in the 

359 


360 


NEW PHYSIOGRAPHY 


bedrock below, and some of it may be supplied by river 
seepage. 

Adhesion Water. — Some showers only “ lay the dust.” 
Others wet the soil grains several inches down. In each case 
the soil below these limits receives no water from the given 
shower, because all of the water that fell is held on the 
surface of the grains of soil by the force of 
adhesion. 

This adhesion water , as it is called, clings to 
the surface of the grains as a film and does 
not fill the pores or spaces between them. 

From this film of moisture the tiny rootlets 
supply vegetation with water. 

In regions of limited rainfall the adhesion 
water is the principal, and often the only form 
of ground water in the region. 

The Water Table. — In moist regions the 
rainfall usually wets the soil as far down as it 
is porous and accumulates on the first im¬ 
pervious layer, filling all spaces between soil 
grains below a certain level. (See Figure 180.) 

The surface below which the ground is satu¬ 
rated with water is known as the water table. 
Above the water table the spaces between the soil grains are 
not filled with water, but with air. 

If a glass cylinder like that shown in Figure 175, or a fruit 
jar, is filled with pebbles which will pass through a sieve with 
a quarter-inch mesh, but which will not pass through a sieve 
with an eighth-inch mesh, these stones will permit a small 
quantity of i nk -colored water to settle quickly to the bottom, 
filling all spaces below a certain level as shown in the lower 
part of the figure. The upper surface of this water is the 
water table in the cylinder. The adhesion water does not 
show in the photograph, but it is easily seen when one has 
the cylinder in one’s hands, particularly where the pebbles 
touch the sides of the cylinder. 



Fig. 175. — A 
Cylinder of 
Gravel Show¬ 
ing Water 
Table 




THE GROUND WATER 


361 


Importance of the Ground Water. — Adhesion water is 
the chief source from which vegetation procures its water. 

The great reservoir of ground water below the water table 
supplies wells and springs. It is the chief source from which 
streams and lakes receive water between showers, thus aiding 
in maintaining a steady flow. 

Distribution of the Ground Water below the Water 
Table. — Ground water, like the run-off, sometimes collects 
in hollows of the impervious layer below it. The underground 



pools thus formed will have a nearly level surface and there¬ 
fore little or no flow, forming “perched water tables” like 
those so frequently found in glaciated regions. 

If the impervious layer slopes, the ground water moves 
slowly through the porous layer, following the slope of the 
impervious layer. This distributes the water laterally; but 
the moving force is gravity, just as it is when it flows on the 
surface. The velocity of the water, however, does not de¬ 
pend upon the slope of the impervious layer, but upon 
the slope of the water table. 

As a rule, ground water follows the shortest path to the nearest 
body of water, such as a river, lake, or sea. 








362 


NEW PHYSIOGRAPHY 


Wells. — If a hole is dug as at A, Figure 176, extending 
below the water table, water will fill the artificial opening to 
the level of the water table, and any water drawn from it will 



Fig. 177. — Artesian Well at Lynch, Nebraska 
Flow, 60 barrels a minute. Photo by Darton, TJ. S. G. S. 


be replaced as long as the water table remains above the 
bottom of the hole. Such a hole is an ordinary well. 

A heavy rainfall raises the level of the water table, and a 



THE GROUND WATER 


363 


continued dry period sometimes lowers it below the bottom 
of the well, causing the well to dry up. As a rule these tem¬ 
porary wells may be made permanent by deepening. 

In Figure 176, B is a permanent well, because its bottom is 
below the level of the dry weather water table, whereas A 
is a temporary well, not deep enough to reach water during 
dry weather. 

Artesian Wells. — In some deep wells the water rises to 
the surface and overflows or is projected into the air like that 
of the well at Lynch, Nebraska (Figure 177). The first well 
of this sort to be described was at Artois near Paris, and from 
the name of this place the word artesian was derived. 



Fig. 178. — Bowl-Shaped Basin of an Artesian Well 


The former belief concerning the rock structure that made 
artesian wells possible was that the water collected in a bowl¬ 
shaped depression like the diagram in Figure 178 in which AB 
and CD are impervious layers, such as clay or shale, separated 
by a porous rock, like sandstone or gravel, that is satu¬ 
rated with water. This layer of water-bearing rock is 
called an aquifer. Two collecting areas are shown by the 
arrows. The water that enters the aquifer cannot escape, 
and it will rise in any well that penetrates the upper 
impervious layer, to the level of the water table in the 
aquifer, WT. 

Artesian wells thus surrounded on all sides by impervious 
strata are rare indeed. Most of them are in inclined rocks 
like those of the coastal plain shown in Figure 179 in which AB 










364 


NEW PHYSIOGRAPHY 


and CD are impervious layers separated by an aquifer. Water 
gathered in the collecting area follows the aquifer toward the 
sea. That the loss of water is not great is shown by the water 
table WT. If water flowed freely through the aquifer, it 
would disappear between showers. The resistance to the 
flow of water through a long layer of sand, or the thinning 


Collecting Area 



AB and CD are impervious layers separated by a porous layer. 


and disappearance of the aquifer, or a fault in the strata that 
closed the space between B and D, would either of them make 
the flow of water more difficult along the slope of the aquifer 
than upward through the well. 

In some artesian wells the pressure transmitted from the 
distant, higher water table is not sufficient to cause the well 
to overflow. The artesian wells of Brooklyn are of this type, 
but as a rule wells are not classed as artesian unless the pres¬ 
sure is great enough to raise the water in the well higher than 
the local water table. 

Artesian well water is usually of exceptional purity, be¬ 
cause the impervious layer keeps local surface impurities out 
and the depth of the well usually insures thoroughly filtered 
water. 

Sources of Contamination of Wells. — Since it became 
known that typhoid fever, diphtheria, and cholera were 
transmitted by drinking water, laws have been passed by the 
legislatures of many states protecting the water supplies of the 
cities. The well, however, upon which it is said that three- 
quarters of the population of the United States depends for 






THE GROUND WATER 


365 


its drinking water, has been sadly neglected by the law¬ 
makers, and through the carelessness or ignorance of prop¬ 
erty owners is doubtless responsible for thousands of deaths 
each year. 

An examination of a very large number of farm wells in 
one of our western states showed that about three-quarters 
of all the wells less than 25 feet deep, and half of those be¬ 
tween 25 and 50 feet deep, contained germs of one or more 
of the diseases mentioned; whereas only one-eighth of those 
between 50 and 100 feet deep contained such germs. All 
wells 100 or more feet deep yielded pure water. 

The average well is a hole in the ground covered with 
planks. Chickens and geese walk over these planks, con¬ 
taminating them. The pumped water runs over, cleans the 
planks, and drips back into the well. 

A trough often stands under the pump spout to which 
cows, pigs and all farm animals come to drink. The filth 
that they leave near the well is often washed into the well 
by the rain. This is carelessness scarcely less criminal than 
is emptying slops on the ground near the well. 


The principal sources of contamination of wells are: sur¬ 
face water, manure heaps, sewers, cesspools, barns, chicken 



Fig. 180.—A Safe Well Located above All Sources of Contamination 


coops, hogpens, laundry drains, swamps, gas works, slaugh¬ 
ter houses, starch works, and certain other industrial plants. 

Sanitation of Wells. — Wells may be unsanitary because 
of improper location with respect to the flow of ground water 
from surrounding sources of contamination or because of 
improper construction. 













366 


NEW PHYSIOGRAPHY 


Location in Porous Soil. — In porous soils the flow of the 
ground water is approximately parallel to the slope of the 
ground and generally follows the shortest path to the near¬ 
est stream. Every accumulation of filth on or near the sur¬ 
face is washed by the rain. The polluted water sinks into 
the ground, becoming a part of the ground water. If the 



Fig. 181. — Depressed Water Table about a Well 

motion of this ground water is toward a well, the well will he 
polluted. (See Figure 180.) 

In cities and towns where barns, outhouses, and cesspools 
are on all sides, unpolluted wells exist only on the upper 
outskirts of the town; but in the open country many wells 
yield satisfactory water. 

If enough water should be drawn from such a well as in 
Figure 181 to lower materially the water in the well, it 

might be contaminated by 
sources below it as shown 
in Figure 181. 

This depression causes 
the water to flow toward 
the well from all directions 
so that the well would be 
contaminated by any filth 
within a distance equal to 
the radius of the depression of the water table. In large 
wells supplying thousands of gallons daily this radius may 
be several hundred feet. 

The so-called artesian wells of Brooklyn are of this type. 


^ u» iA 

==a 


D A 

vm^Lr,. 


Fig. 182. — The Mason Well 

















THE GROUND WATER 


367 


Construction of Wells. — The construction of a type of 
well that has proved very satisfactory is shown in Figure 
182. It is lined with brick or stone, laid in cement. It has 
a cement or stone top with an iron manhole in it. An apron 
of cement with a radius of about ten feet surrounds the well, 
and a tile drain carries away the waste water. 

The following general laws of the sanitation of wells should 
never be violated: 

1. Locate a well so that the natural flow of the ground water cannot 
bring filth into it from any source whatever. 

2. Construct a well so that no water can get into the well at any 
point above the water table. 

Springs. — Ground water is constantly in motion, flow¬ 
ing through porous layers of rock or through fissures and 



caves in impervious rock. The motion is quite swift in caves 
and large fissures; but when flowing through fine porous 
layers, the rate is only a few feet a day. 

When water comes to the surface through a natural opening , 
it is called a spring. 

Hillside Springs. — If an impervious layer underlies a por¬ 
ous layer as shown in Figure 183, a line of hillside springs may 
be formed where the impervious layer, HS, crops out on the 
side of a hill. Figure 184 shows a very unusual example of 
hillside springs in the great volume of water. The Thousand 
Springs form quite good-sized falls as the water from them 
flows over the side of the Snake River canyon in Idaho. 





368 


NEW PHYSIOGRAPHY 



Fissure Springs. — Ground water frequently comes to the 
surface through fissure in the bedrock, forming what is called 
a fissure spring. In some instances the water comes from 
the local ground water which sinks into the joints of an im¬ 
pervious rock and follows these joints to the side of some 
valley where the joints come to the surface. 


Fig. 184. — Thousand Springs, Snake River Canyon, Idaho 
Photo by I. C. Russell, U. S. G. S. 

In other instances the water comes from a distance beneath 
an impervious layer and reaches the surface through a fault. 
The structure of such a spring is practically identical with 
that of an artesian well, except that no part of the spring 
is artificial. (Figure 185.) 

Springs coming to the surface from beneath an impervious 
layer supply water that has been protected from contamina¬ 
tion from local sources. If the water is “hard,” contain¬ 
ing dissolved minerals, if it has a temperature 10° or more 
below that of the local subsoil, and if the supply of water is 




THE GROUND WATER 369 

well sustained during dry seasons, it is probable that the water 
comes from some deep-seated source and is wholesome. 

Mineral Springs. — Certain springs, like those at Sara¬ 
toga, New York, and at Vichy in France contain carbon 
dioxide in solution, which causes them to effervesce as the 
water flows from the spring. Others, like the White Sulphur 
Springs of Virginia and the line of sulphur springs near the 



Fig. 185.— Spring from Cave Formed by a Stream Following a Fault 


Finger Lakes in New York State, contain hydrogen sul¬ 
phide, an ill-smelling gas, in solution. 

Nearly all springs coming from deep-seated sources con¬ 
tain much dissolved mineral matter which often gives the 
water valuable medicinal properties. Several hundred such 
springs are known in this country, notably in New York, 
Michigan, and Wisconsin. 

Temperature. — In summer, well water is noticeably 
cooler than surface water, and some spring water has a tem¬ 
perature within a few degrees of the freezing point the year 



370 


NEW PHYSIOGRAPHY 


round. Numerous towns derive their names from the no¬ 
ticeably low temperature of their water supply, giving us 
towns called “Coldwater” and “Cold Spring” in almost 
every state. 

Great springs of boiling water occur in Yellowstone Park, 
and well-known hot springs are found in Arkansas, Tuscany, 
and other regions giving evidence of volcanic activity in 
the past (Figure 192). 

Quicksands. — Springs rising through very fine sand 
form the dangerous quicksands in which animals are some¬ 
times engulfed. 

Geysers. — Geysers are chiefly important because of the 
interest that they arouse in tourists. They are boiling springs 

from which hot water and steam 
are thrown into the air from 
time to time. They occur only 
in regions giving evidence of 
recent volcanic activity, such 
as Yellowstone Park, New 
Zealand, and Iceland. The 
cone of a geyser consists of an 
opaque white form of silica 
called geyserite, page 378, which 
is deposited around the end of 
the tube from which the water 
and steam are ejected. 

The first symptom of an 
approaching eruption is the appearance of steam rising 
gently from the cone. Soon the water in the tube rises and 
falls spasmodically, with rumbles and growls like those that 
are produced when a jet of steam is condensed in cold 
water. The vigor of the disturbance increases gradually 
until, finally, a great stream of boiling water accompanied 
by much steam is thrown into the air. The Giant Geyser 
of Yellowstone Park throws its column to a height of 250 
feet and plays for an hour or more. 







THE GROUND WATER 


371 


The following facts concerning the eruption of a geyser are 
well established: 

1. The tube of the geyser extends down to a large mass of hot 
rock. 

2. After an eruption, the tube is filled with cold water from the 
ground water or, perhaps in part, from water of a preceding eruption 
that returns to the tube through crevices or porous rocks. 

3. The temperature at which water boils is increased by pressure 
acting upon it. At a pressure of one atmosphere water boils at 
212° F., at two atmospheres at 250° F., and at eight atmospheres 
at about 350° F. 

4. Experiment shows that liquid pressure increases at the rate 
of one atmosphere for each 34 feet in depth. Therefore 34 feet 
below the surface of the water in the tube the pressure would be two 
atmospheres and the boiling point of the water would be 250° F., 
and 240 feet below the surface it would be just under 350° F. 

Figure 186 is a diagram of a geyser tube extending to hot rock. 
WT is the water table of the region, and the depth is assumed to be 
such that the boiling point of water at the bottom of the tube is 
300° F. 

At the water table the boiling point is, of course, 212° F. 

When an eruption ceases, the cold water at the bottom of the tube 
is soon heated to its high boiling point. Bubbles of steam rise into 
the water above, condensing, causing the agitation and rumbling 
that occur whenever live steam escapes into cold water, warming 
the layer above to its boiling point, and increasing the supply of 
steam. 

Eventually the flow of steam becomes so great that some of the 
water in the tube overflows, diminishing the pressure below and 
lowering the boiling points all along the tube. The water is then 
hotter than its boiling point and a great volume of steam is instantly 
produced that drives the water out of the tube violently. 

The Excelsior Geyser when in eruption raises the level of 
Fire Hole River seven feet in a few minutes, a fact which 
shows the great size of the underground reservoir that it 


372 


NEW PHYSIOGRAPHY 


empties and which also indicates that most of the water 
comes from below the water table. 

Destructive Work of Ground Water. — The chief proc¬ 
esses by which ground water acts upon earthy matter are 
solution, oxidation, and stream erosion. 

Solution. — Pure water has little power to dissolve earthy 
matter, the only important kind of bedrock dissolved by 
it being rock salt. Ground water, however, is rarely pure; 
it dissolves oxygen and carbon dioxide from the air while 
falling as rain, and each of these gases increases its action 
on the rocks. Water containing carbon dioxide dissolves 
limestone, marble, calcite, and several other substances 
found in the earth. 

Water containing minerals in solution is called mineral 
water or sometimes hard water. It destroys the efficiency 
of soap until all of the mineral matter has been removed 
from the water by the soap, or by other means, after which 
the water is said to be soft. 

Water containing alkali in solution dissolves quartz; and 
hot water, especially when under great pressure, dissolves 
many minerals that are dissolved with great difficulty, or 
not at all under ordinary conditions. 

Oxidation. — This process depends upon the activity of 
the oxygen dissolved by the rain and carried into all porous 
rock not protected by an overlying impervious layer. The 
oxidation thus caused differs from weathering by oxygen in 
that it extends as far below the surface as the water pene¬ 
trates. It is the cause of the rusty streaks seen in exposed 
masses of bedrock. 

Stream Erosion. — When water flows through fissures or 
caves, friction increases the rate at which the bedrock is 
disintegrated. By the joint action of these processes the 
great caves of the world have been formed. 

Caves. — Figure 187 shows surface streams entering 
fissures in the bedrock at 1 and 3 respectively. The fissures 
end at the lower impervious layer, 4-5, along which the 


THE GROUND WATER 


373 


water has found a passage between the layers, flowing out 
at C to form a spring. This was doubtless the beginning of 
such caves as that shown in Figure 188. 

In Figure 188 the openings in the bedrock have become 
sink holes marked B, and the narrow passages have become 
great “ amphitheaters ” marked C. 

Mammoth Cave. — One of the largest known caves in the 
world is Mammoth Cave, Kentucky. It is over nine miles 



Fig. 187. — The Beginning of a Cave 



Fig. 188. — Cave Formed in Limestone by the Ground Water 


from entrance to farthest recess. It has a network of numer¬ 
ous galleries and passages which cross and recross one another, 
with a total length of more than two hundred miles. It 
has its own rivers and lakes, in which are found sightless 
crayfish. Countless bats cling to its walls. Its blind rats 
have very long sensitive whiskers for feelers. The many 
blind forms of animal life are probably the descendants of 





































374 


NEW PHYSIOGRAPHY 


normal animals that entered the cave long ago. Skeletons 
of men have been found there. 

The best preserved skeletons of prehistoric man and the 
best samples of his hand work have been obtained from the 
limestone caves of France, Belgium, and Spain. 

In limestone regions the surface streams sometimes form 
sink holes that lead to underground passages. Where sink 

holes are numerous there may 
be no surface streams, as in 
the Karst region east of the 
Adriatic Sea. Occasionally, 
too, portions of the roof of a 
cave fall in, leaving a portion 
that forms a natural bridge. 
Natural bridges are also 
formed in other ways. 

Underground streams do the 
same kinds of work that the 
surface streams do. In addition 
to this, some of them, where 
they are closely confined, wear 
away the roof above them. 

As solution of the rock walls of the cave goes on, the pas¬ 
sages through the limestone grow larger and larger, and the 
roof of the cave becomes thinner and thinner. When all of 
the roof of the cave, as R, Figure 189, falls in, the cave be¬ 
comes a valley or a canyon; but if a portion of the roof holds 
its place when the rest falls in, it becomes a natural bridge 
across the valley, as shown in Figure 190. 

Small portions of the roof of a cave, which remain in position 
after the rest of the roof has fallen in, are called natural bridges. 

Constructive Work of the Ground Water. — Water cir¬ 
culating through joints, fissures, and caves in the bedrock 
often reaches a great depth where the temperature is high 
and the pressure great. Thus it develops a great solvent 
power. Returning toward the surface, it undergoes several 



Fig. 189. — Valley Formed from 
a Cave 



Fig. 190. — A Natural Bridge 












































THE GROUND WATER 


375 



Fig. 191. — Natural Bridge in Sandstone, South Dakota 









376 NEW PHYSIOGRAPHY 

changes, each of which tends to cause deposition of mineral 
matter held in solution: 

1. Cooling causes the deposition of those minerals that are more 
soluble in hot than in cold water. 

2. Diminishing liquid pressure causes deposition of minerals that 
are more soluble under high than under low pressures, but the 
principal effect of diminished pressure is due to the escape of dis- 


Fig. 192. — Deposits from Hot Springs, Yellowstone National Park 

solved gases which causes the deposition of limestone in various 
forms. 

3. Complete evaporation of the water on emerging causes deposition 
of all the dissolved mineral. 

4. Partial evaporation will cause deposition as soon as the water 
becomes saturated with a mineral. In fact, several minerals are 
sometimes found in layers, with the mineral deposited when the 
smallest percentage of the water was evaporated at the bottom. 

Materials Deposited. — The principal materials deposited 
by ground water are limestone ( travertine ), alkali, silica, and 
cal cite. 



THE GROUND WATER 


377 


Travertine. — As described on page 316, travertine is de¬ 
posited by ground water in several forms: calcareous tufa, 
the loose or porous 
variety deposited by 
springs (Figure 143); and 
the denser varieties, 

Mexican onyx and stalac¬ 
tites (Figures 145-146). 

These forms of traver¬ 
tine are usually deposited 
by cold hard water where 
it emerges from the 
ground as a spring. It is 
also deposited by hot 
springs in the interesting 
form shown in Figure 
192 of the terraces of 

the mammoth hot springs of Yellow¬ 
stone Park. 

It is believed that these deposits 
are aided if not caused by the presence 
of algae in the hot water. 

Alkali deposits that unfit the soil for 
agriculture are often formed in arid 
regions by the evaporation of ground 
water that rises to the surface. In 
some instances the alkali may be re¬ 
moved by flooding the ground with 
water. 

Ores of gold, silver, lead, copper, and 
other metals were deposited on the walls 
of the passages through which the 
ground water circulated, forming veins. 
Quartz, calcite, and several other min¬ 
erals are sometimes deposited with the ores often completely 
filling fissures and cavities of the rock with parallel layers of 



Fig. 193A. — Section of a 
Fissure Vein 

Photo by Arey. 



Fig. 193. — Agate Showing Structure 
Photo by Arey. 







378 


NEW PHYSIOGRAPHY 


minerals. Figure 193A shows a portion of a fissure thus 
filled. 

The agate, Figure 193, was a cavity in the rock that was 
filled by successive layers of silica. These layers were given 
different colors by different metallic oxides. 

Geyserite, a form of silica, is deposited in and about the 
crater of a geyser as explained on page 370. 

Summary. — Solution and deposition by ground water 
tend: (1) to collect minerals that were widely scattered so as 
to form valuable veins of ore; (2) to bring minerals from 
far below the surface to points within easy reach of the miner; 
(3) to repair breaks and fill cavities in the bedrock; (4) to 
cement the particles of mantle rock together forming con¬ 
solidated rock. 


Deposits Formed by Ground Water 


Causes 

Location 

Material 

Form 

I 

Evaporation 

(A) Surface of soil 

Alkali 

Encrustation 

(B) Bottom of lake 

Gypsum, salt, 
etc. 

Stratified 

II 

Changes in tem¬ 
perature and 
pressure 

(A) Cavities and fis¬ 
sures 

Ores, silica, cal- 
cite, etc. 

Veins 

( B ) Porous rock 

Silica, calcite 

Cement 

(C) Geysers 

Geyserite 

Craters and 
cones 

III 

Loss of carbon 
dioxide 

(A) Ceiling of cave 

Travertine 

Stalactite 

( B ) Ground of cave 

Travertine 

Stalagmite 

( C ) About springs 

Calcareous tufa, 
Mexican onyx 

Strata 

IV 

Chemical action, 
precipitation 

(A) Fissures 

Ores, sulphides, 
etc. 

Veins 

( B ) Porous rocks 

Calcite, silica, 
iron, etc. 

Cement 

V 

Algce 

About hot springs 

Travertine 

Terraces and 
sheets 









































THE GROUND WATER 


379 


Relation of the Level of the Water Table to Agriculture 

The depth of the water table at a given point depends 
upon: (1) the amount of rain water that enters the ground, 
(2) the amount of surface evaporation, (3) the depth of the 
first impervious layer, (4) the slope of the water table, 
(5) the resistance offered to the flow of water through the 
mantle rock. 

As should be expected, when so many conditions modify 
it, the level of the water table varies greatly from place to 
place with marked influence upon the usefulness of the region 
for agriculture or habitation. If the water table is above the 
surface, lakes and ponds which often cover much valuable 
soil occur. If the water table is at, or slightly below, the 
surface of the soil, swamps and marshes occur. These 
have only limited usefulness. Wet land of this type may be 
reclaimed by drainage. If the water table is too low, an 
arid condition obtains, and agriculture is impossible except 
where water can be obtained for irrigation or where the 
methods of dry-farming can be carried out. 

Water Table Too High. — The wet lands of the United 
States are for the most part east of the Mississippi Valley. 
Minnesota is said to have some 8000 lakes, about half of 
which will become farm lands by natural processes within 
half a century. Connecticut has about fifteen hundred lakes, 
not including those that have been reclaimed, and Wis¬ 
consin and Michigan each have many more. 

Shaler says that there are 64,000,000 acres of swamp land 
between the Appalachian Mountains and the Atlantic coast 
that can be reclaimed. 

There are also hundreds of square miles of swamp in the 
region north of the Ohio and the Missouri Rivers and a very 
large area about the delta of the Mississippi River. It is 
maintained that one-sixth of the state of Arkansas is alluvial 
and that this ground may be quite easily reclaimed. When 
drained, these lakes and swamps will be among our greatest 
natural resources. 


380 


NEW PHYSIOGRAPHY 


There is much farm land under cultivation that is too wet 
to produce profitable crops; indeed it is maintained that 
the drainage of soils which are too shallow, or of bad texture, 
or but slightly wet, adds more to the value of farms and to 
the profits of the farmer than is derived from the drainage 
of lakes and swamps. 

As a rule, inspection of a growing crop will tell us whether 
a given field or section of a field needs drainage. Plants on 
such land are always late in starting and never have the 
vigorous healthy growth of plants on neighboring well- 
drained land. The effect really depends upon the depth of 
the water table. 

In Holland it is prescribed that on pasture lands the water 
table shall be kept from one to one and one-half feet below 
the surface and that on land used for general farm crops it 
shall be kept from two and one-half to three and one-half 
feet below the surface. 

There are several other important and advantageous 
results from proper drainage of land, besides removing the 
excess of water and making the ground firm enough to drive 
on. 

1. Drainage ventilates the soil, renewing the supply of oxygen 
which the plants require. 

2. It increases the area of the film of adhesion water that covers 
the grains of soil and from which alone the roots of plants can draw 
their water supply. 

3. It makes the soil earlier and warmer by removing many 
pounds of water from the soil. 

4. It increases the healthfulness of the region by eliminating the 
mosquito. 

Water Table Too Low. — There are a few regions where 
the water table is so near the surface that crops are entirely 
independent of local rainfall. 

The section of Holland that has been reclaimed is probably 
the most important region of this type in the world. There 
may be a few other regions on certain flood plains and deltas 


THE GROUND WATER 381 

that have similar advantages over regions that are subject 
to drought during the growing season. 

It has been stated that 65 per cent of all the land of the 
earth has too little rainfall and a water table too low for 
successful agriculture with ordinary methods. Much of the 
remaining 35 per cent, known as the humid section, has the 
water table so low that it is dependent upon frequent rains 
during the growing season to prevent loss of crops. 

Irrigation. — For many centuries man has raised farm 
crops in arid regions in which water could be procured for 
artificial watering. The early Egyptians pumped water 
from the Nile by means of the shadoof, as early as 3400 b.c. 
and used the water for irrigation. The ancient civilizations 
in the valley of the Tigris and the Euphrates also depended 
upon extensive systems of irrigation. In the Americas it 
was first practiced by the Indians of Arizona and New 
Mexico and by the ancient Peruvians and Bolivians. 

Irrigation systems cost large sums of money; but when 
completed, they bring two important advantages: an in¬ 
crease in the value of the land and an increase in the number 
of inhabitants a square mile who can make a living from the 
land. In Oregon very cheap land was changed to orchards 
worth $1000 an acre. In some of the well-developed irriga¬ 
tion districts, both in China and in America, populations 
in excess of 500 a square mile have been supported. 

It is said that farms under irrigation have other advantages 
over farms in the humid regions besides the freedom from 
loss by drought, in that they are not subject to the many 
cloudy days that accompany the showers of the humid sec¬ 
tion, and therefore that the fruit raised will be of better 
color and of higher flavor than that of the humid section. 
This conclusion, however, is questioned. 

There are about 170,000,000 acres under irrigation in the 
world, the largest area being in India and the next largest in 
the United States. 

The Roosevelt dam, Figure 194, built across a canyon 


382 


NEW PHYSIOGRAPHY 



Fig. 194. — Roosevelt Dam 

Salt River Irrigation Project. Courtesy of the U. S. Reclamation Service. 














THE GROUND WATER 


383 


in the Salt River of Arizona forms a lake with an area of 
25.5 square miles, that is 1080 feet wide at the dam and 
about 300 feet deep in places. It is estimated that the water 
in this great reservoir will irrigate nearly 300 square miles of 
farmland. On the right 
of Figure 194 we see a 
bridge across the canal 
that carries the water 
to the farms. In the 
valley below the dam is 
a powerhouse that pro¬ 
duces light and power. 

About four tenths of 
the United States is 
too dry to produce 
crops without artificial 
watering. Even in the 
eastern states there are 
many seasons when 
irrigation would be a 
profitable investment. 

The map, Figure 
195, shows in black the 
land that may be irri- Fig. 195 . — irrigation center op the west 

gated by works built by The black portions show the land irrigated by the 
works the Government has built or is now building 

the government and in¬ 
cidentally, by comparison with the white area of the map, the 
relatively small proportion of the arid land that is benefited. 

It is estimated that when all the irrigation projects now 
contemplated by the United States Reclamation Service 
shall have been completed, there will be about 45,000,000 
acres of land under irrigation and that they will maintain 
about 45,000,000 inhabitants. 

If all the water that falls on our arid region were to be 
used for irrigation, it would be sufficient to irrigate only 
10 per cent of the arid land. 











384 


NEW PHYSIOGRAPHY 


Dry-farming. — In its literal meaning dry farming means 
farming without water, which is a manifest absurdity. When 
printed as a compound word, dry-farming has come to mean 
farming in arid or semiarid regions without irrigation , by 
methods that preserve the limited rainfall for use during the 
growing season. Considerable progress has been made in 
this direction, as is indicated by the following statement by 
one of the leaders in the movement: “ Crop failures due to 
untimely frosts, blizzards, tornadoes, or hail may perhaps 



Fig. 196. — Map op the Area Where Dry Farming Must Be Practiced 


be charged to Providence, but the dry-farmer must accept 
any responsibility for any crop injury from drought.” 

There are in the United States more than 550,000,000 
acres of land that may be successfully used for dry-farming 
but that cannot be reclaimed by any other known process. 

The limited rainfall west of 96° west longitude would not 
as a rule be sufficient to produce a crop every year even if 
it all fell during the growing season. In arid regions all of 
the rainfall is held as adhesion water within a dozen feet of 
the surface; and if surface evaporation can he prevented , the 
water can be held there without loss almost as perfectly as 
it could be kept when sealed in a fruit jar. 















THE GROUND WATER 


385 


One of the important measures adopted by the dry-farmer 
is constant cultivation so that a layer of dry dust forms over 
the surface. This layer, called a dust mulch, is composed of 
particles that are free from adhesion water, and it therefore 
prevents evaporation. The method is so successful that 
practically the whole of the rain, it has been proved, can be 
stored in the ground for two or more years. In certain regions 
the rainfall must be so stored for two years to accumulate 
enough to produce a crop, and in such regions the land must 
lie idle every other year. 

Four simple rules guide the dry-farmer in his work: 

1. Keep the surface soil loose and crumbly so that all of the 
rainfall may sink into the ground. This requires frequent culti¬ 
vation. 

2. Keep the surface soil as dry as the dust in the road in the 
summer, thus preventing evaporation. This requires frequent 
cultivation. 

3. Select drought-resisting crops. 

4. In'regions of limited rainfall (10 to 15 inches) a crop should 
not be raised oftener than every other year, and the surface of the 
soil should be kept loose, crumbly, and dry— as directed in No. 1 
and No. 2 above — during the year that the land is idle as well as 
when a crop is being produced. 


QUESTIONS 

1. What are the principal reasons that you have for believing 
that the water supply of your town is not contaminated? 

2. Why is the water table not level? Would it be more nearly 
level in sand, in gravel, or ordinary soil? Why? 

3. What angle is formed by the surface of a lake and a plumb- 
line? By the surface of a river and a plumb-line? Why this dif¬ 
ference? 

4. Illustrate by means of a diagram properly labeled the position 
of the water table and a spring, a permanent stream, a temporary 
stream, a marsh, a lake, and an artesian well. 

5. Show the proper relative positions of a house, barn, well, 
and outbuildings on a side hill. Give your reasons. 


NEW PHYSIOGRAPHY 


386 

6. Why do cities generally depend for their water supply upon 
lakes or rivers instead of upon wells ? 

7. How may geysers choke themselves until they no longer 

erupt? . _ .... 

8. How will the evaporation of water, furnished by irrigation, 
affect the amount of soluble plant food in the soil below the surface 
and at the surface? 

9. “What’s the use of cultivating corn when there are no 
weeds in it?” 

10. State the special conditions that cause (a), the formation of 
caves, ( b ) veins, (c) dikes, (d) natural bridges. 

11. State the chief mechanical and chemical effects of ground 
water. 


CHAPTER XX 


RIVERS 

Rivers as Highways. — As civilization spreads to unknown 
lands, explorers find great forests covering all regions having 
sufficient rainfall to support them, except in the vicinity of 
the poles. The eastern 
part of North America, 
for example, was a dense 
forest crossed only by 
trails along which the 
Indians traveled in single 
file, because the trails 
were too narrow to allow 
them to do otherwise. 

Some of these trails, 
doubtless, may have 
allowed travel on horse¬ 
back by the earliest 
settlers, but there were 
no wagon roads. 

The principal highway 
used by the Indian was 
the river, and the direc¬ 
tion of travel, explora- 

. Fig. 197. — Illustrating the Influence of 
tion , and settlement by Rivers on exploration 

white men was con¬ 
trolled by the locations and the courses of the rivers. 

The extent to which river highways expedite exploration is 
well illustrated by the relative progress of the French and 
English explorers of North America. The English who 

387 


























388 


NEW PHYSIOGRAPHY 


settled at Jamestown east of the Appalachian Mountains 
found only short rivers, and in 150 years extended their 
domain only as far as these rivers enabled them to travel. 

The French, however, settling at the mouth of the St. 
Lawrence, quickly occupied the whole of the St. Lawrence 
Basin, including the Great Lakes, and crossed the height of 
land that separates it from the Mississippi Basin, at various 
“ portages ” marked X in Figure 197. Each portage led 
to a tributary of the Mississippi and made it possible to 
reach any part of the great Mississippi system with its 
thousands of miles of navigable water. 

In 1750 the French claimed most of the territory between 
the lines AB and CD in Figure 197, although they had only 
about 80,000 inhabitants in the region; whereas the English 
with a population almost 20 times as great were confined to 
the area between the line AB and the coast. 

Comparison of the census map of 1790 (Figure 198) 
with that of 1820 (Figure 199) shows that our population 
advanced during that interval along the Ohio and Cumber¬ 
land Rivers to the Mississippi, and thence south to New 
Orleans and north to Quincy, Illinois; it indicates also 
that population followed the Missouri River to Kansas 
City, showing that the settlers used the rivers as highways. 

Many explorers besides the French used the rivers of 
North Americas as highways. The Spanish came up to 
Santa Fe from Mexico along the Rio Grande; the Fremont 
Expedition of 1842 followed the South Platte, and the Lewis 
and Clark expedition followed the Missouri, the Yellowstone, 
and the Columbia. 

Besides the excellent highway for boats which rivers pro¬ 
vide, many river valleys furnish a graded location for wagon 
roads and railroads that engineers find better than any other 
location in the region. 

As a rule, streams also control the location of roads that 
cross the stream, limiting the crossing to shallow places 
that can be forded or narrow places that can be bridged 


RIVERS 


389 


cheaply; and in the case of canyons or steep-sided valleys, 
to gaps in the valley wall. The “ Spanish Trail,” in 
western United States, crossed the Green River of Utah 
where there is a gap in the canyon wall, and our railroads 
now use the same gap. 

Where there is sufficient travel to warrant the expense of 
a bridge, neither the deep river, the rapids, nor the canyon 
is an insurmountable barrier. 

Rivers and Commerce. — Just as the canoe was the chief 
means of travel in North America in the early days, so the 
flat bo at was the chief means of transporting freight. The 
river was the first artery of commerce in all countries. 
Ancient histories tell us of boats propelled by oars, poles, or 
sails, carrying cargoes on the Nile and on the Euphrates. 

The transportation of commodities downstream on the 
Mississippi became so important in the early days of the 
last century as to lead toward the “ Louisiana Purchase.” 
The journey upstream with a flatboat was slow and difficult, 
sometimes lasting several months; but the boats could carry 
many tons of freight, required small crews, and traveled 
downstream without power at the rate of four or five miles 
an hour. 

The appearance of the steamboat on the Ohio River in 
1811 decreased the time required for upstream journeys to 
about 5 per cent of that formerly required, and caused a 
great increase in the volume of freight carried on the river. 
As a result, freight charges were reduced to about one-fourth 
of the former rates. 

When railroads were built, transportation by boat in 
America began to decline, because (1) river transportation 
is slower than rail transportation; (2) it is often obliged to 
follow roundabout routes; (3) it must be suspended in the north 
in winter; (4) it is suspended during the summer on many 
streams because of low water. 

Even with these disadvantages, river transportation has 
exercised a salutary influence in controlling the average rate 


390 


NEW PHYSIOGRAPHY 



Fig. 198. — Showing the Distribution of Population in 1790 


































RIVERS 


391 























































392 


NEW PHYSIOGRAPHY 


per ton-mile for the transportation of freight. In 1837 the 
average charge made by railroads was seven and one-third 
cents per ton-mile. It had dropped to less than one cent 
per ton-mile by 1905. Railroads are now carrying the pas¬ 
senger and express traffic of this country, including all per¬ 
ishables and all light articles; but they cannot compete with 
the river transportation companies in handling heavy or 
bulky freight, especially when carried downstream. 

In 1925 the Ohio River and its tributaries carried about 16,- 
000,000 tons of coal, lumber, gravel, and sand, and more than 
75 per cent of it was carried downstream. On the Mississippi, 
coal, lumber, and sand are carried downstream; and some of 
the products of the delta regions, such as cotton, sugar, rice, 
and petroleum, are carried upstream. 

Exceptionally favorable conditions for transportation on 
the Great Lakes have enabled the companies operating there 
to control much of the freight business of the region through 
the low cost of transportation. In 1909 the through rate for 
iron ore from Lake Superior was less than one mill per ton- 
mile whereas the rate by rail was one cent per ton-mile. 

The “ Soo Canal ” around the falls of St. Mary (Figure 230) 
makes it possible for boats to pass from Lake Superior to 
Lake Huron; and although the canal is closed for about five 
months every year, it carries about four times as much freight 
during its seven months period as the Suez Canal carries 
during its twelve months season. Eastbound freight through 
the “ Soo ” is principally iron ore and grains; westbound 
freight is principally coal. Much lumber, however, comes 
through the “ Soo,” and much more is shipped from the 
shores of Lake Michigan and Lake Huron. 

River Transportation in Europe. — The rivers of Europe 
have been “ corrected ” and improved at great expense and 
are of much greater relative importance in comparison with 
railroads than are the rivers of the United States at the pres¬ 
ent time. This is partly due to the greater relative mileage 
of railroads in this country, but there is no doubt that busi- 


RIVERS 


393 


ness will be greatly stimulated and the cost of manufactured 
articles greatly diminished when the thousands of miles of 
navigable rivers and canals in the United States are devel¬ 
oped as highly as those on the continent of Europe are at 
the present time. 

Rivers and Water Supply. — Man has always gone to 
streams for a large part of his water supply, both for house¬ 
hold purposes and for irrigation. Practically all rivers fur¬ 
nish water suitable for irrigation, but great care must be 
exercised to secure water suitable for drinking purposes, for 
the following reasons: 

1. Streams that are safe for drinking at their ordinary stage 
become unfit for drinking when at flood stage because of filth washed 
into them from the land. 

2. Water that appears and tastes as though it were absolutely 
pure may be loaded with the germs of certain contagious diseases. 

3. Sewage may be discharged into the stream above the point at 
which the water was taken. 

Many towns empty their sewage into a river, thus endan¬ 
gering all towns that draw water from the river below them. 
Notable instances of this practice are found at many points 
on the Great Lakes and on the Mississippi River. Where 
most of the cities depend upon the river or the lake for their 
water supply and empty their sewage into the same body of 
water at a point some distance below their waterworks in¬ 
take, such conditions always oblige cities to purify their 
water before it is safe to use. Most of the cities on the 
Mississippi go to the river for their water supply, and so do 
some of the cities on the Hudson, Connecticut, and other 
rivers. 

New York City derives most of its water supply from 
streams. The old Croton system depended upon the Croton 
River Basin, and the new Catskill system upon the upper 
portion of the basin of Esopus and Schoharie Creeks and 
some near-by streams. 


394 


NEW PHYSIOGRAPHY 


It is obvious that densely populated stream basins are 
more likely to contain impure water than sparsely populated 
basins, and many states give their cities sanitary control over 
the basin from which they draw their water. In spite of such 
control, New York City has found it increasingly necessary 
to purify the water from the Croton system with chlorine, so 
as to prevent the recurrence of epidemics of typhoid fever. 

Military Advantages. — The feudal castle was surrounded 
by a deep moat as a means of defence. Modern armies fre¬ 
quently select positions with a natural moat; i.e ., with a 
river between them and the enemy. The degree to which a 
stream aids an army depends upon the characteristics of the 
stream. A frozen stream, a dry stream, or a shallow stream 
with a wide, open valley would only slightly retard the ad¬ 
vance of an enemy. A deep river would require bridges, and 
these might be destroyed by artillery as fast as an enemy 
could rebuild them. 

A canyon like that of the Colorado and a gorge like that 
of the Niagara, or rapids like those near Niagara Falls, would 
be practically impassable for an attacking army and would 
prevent a frontal attack upon an army defending them. 
Many illustrations of this use of rivers are to be found in the 
history of wars. In the Battle of New Orleans, 1815, the 
Mississippi, and in the World War, the Marne, the Aisne, 
the Dnieper, the Tagliamento, and the Piave were thus used. 

Rivers as National Boundaries. — The fact that rivers are 
easily described in deeds and treaties has led to their wide use 
as national boundaries. In our own country the Mississippi 
was once the western boundary; the Rio Grande is our present 
boundary for some distance on the south and the St. Lawrence 
on the north. The Missouri River separates Nebraska and 
Kansas from Missouri and separates Iowa from South Da¬ 
kota. In neither of the instances mentioned, excepting per¬ 
haps that of the St. Lawrence, has the river proved to be a 
satisfactory boundary, because of the tendencies of the rivers 
concerned to shift their channels. There have been contro- 


RIVERS 


395 


versies between several states bordering the Missouri and 
Mississippi Rivers and between the United States and Mexico 
because of the shifting channel. 

The only satisfactory boundary line for a nation is one 
possessing military advantages like a canyon or a mountain 
range. The Niagara River, from the Falls to Lewiston, is an 
ideal national boundary; its channel does not shift, it pre¬ 
vents smuggling, and in case of war it would need no defence 
except where the gorge was bridged. 

Sources of River Water. — (1) During rainfall some water 
falls directly into the river. (2) During and for a short time 
after a rainstorm, a portion of the water precipitated flows 
over the land into some body of water. This portion is most 
important, but it is only a temporary source. (3) Some water 
is supplied by springs and by seepage from the soil. This 
action is continuous and is the only source of supply between 
showers. It is this source that makes streams permanent. 

The portion of the rainfall that remains upon the surface 
is called the run-off. It accumulates in hollows forming ponds 
and lakes, or flows through depressions forming streams. 

On level fields rain water at first flows over the ground in 
sheets; but some parts of the soil are worn away more rapidly 
than others so that shortly we find the water in the low 
places, and thus a stream is formed. 

This action is well shown, on a small scale, on ocean beaches 
at low water. The water table in the adjacent land rises as 
the tide rises; and as the tide falls, sea level is soon below 
the water table. Ground water then runs out on the beach. 
At first the water runs down the sloping beach as a sheet, but 
farther down we find it in little valleys which join others, as 
they grow larger, until finally we have a trunk stream. The 
whole system now resembles a tree with many branches. 

Hollows in the path of this stream become ponds, and 
the deltas which form in them quickly fill the pond with 
sand; steep places form rapids; and a board embedded in 
the sand in the path of the stream may form a waterfall. 


396 


NEW PHYSIOGRAPHY 

These miniature streams, or rills, as they are sometimes 
called, are very perfect: imitations of the early stages of our 
large streams. They may be seen on almost any of our gently 
sloping beaches of fine materials, during the later hours of 
ebb tide. 

Definitions. — Rivers , creeks, and brooks are streams of 
water flowing through natural channels toward a lower level. 
They differ chiefly in size. 

The largest stream in a region is generally called a river, 
and the smaller streams are called creeks or brooks; but what 
is called a creek in one vicinity may be much larger than some 
rivers in other parts of the country. 

A river and its tributaries form a river system, and the land 
drained by the river system is called the river basin. Since 
water flows downhill, it follows that the source of a river is 
higher above the mouth of the river than any point in the 
river bed between the source and the mouth. As a similar 
fact applies to every tributary, we see that a line from the 
mouth of the main stream, surrounding all of the tributaries 
of the stream, encloses a shell-like depression called a basin. 

The rim of the basin is called a divide, because it separates 
the basin of one stream from that of its neighbor. 

Every tributary as well as the main stream is engaged in 
the work of wearing away the land and flows along its bed 
in the depression that it has formed. This depression is the 
river valley. The channel is the line following the deepest 
part of the stream and is marked by the line of swift current. 

Drainage. — As the quantity of water received by a stream 
increases, it cuts larger and larger channels and so in time 
adjusts itself to the average volume of water that it has to 
carry. The land near the stream is satisfactorily drained so 
long as the average volume is not greatly exceeded. 

Floods. — It sometimes happens that the rapid melting of 
winter snows or that heavy and long continued rainfall over 
a large portion of a river basin supplies more water than 
the river’s channel can carry, so that it overflows its banks, 


RIVERS 


397 



destroying much property and often drowning many people. 
This is especially true of the great rivers of China. Fig¬ 
ure 200A is a photograph of the Ohio River at flood stage 
taken at New Albany, Indiana. 

As the water rose, the inhabitants had to move out of the 
lower stories of their houses and in some instances had to 
abandon them entirely. They had to come and go in boats 


Fig. 200A. — The Ohio Rivek at Flood Stage 
New Albany, Ind. The streets of the town are under water, and boats replace motors. 

instead of autos and swimming was better than walking on the 
sidewalks. In the flood shown the loss of life was not so large as 
it has been in many floods; but much property was damaged, 
some houses were carried away, some were wrecked, and all 
of them were injured by the sediments deposited in them. 

Among the most disastrous floods are those of the Yellow 
River of China. In 1897, 50,000 square miles of its flood 
plain were inundated, covering many villages. More than 
1,000,000 people were drowned, and an equally great loss 





398 


NEW PHYSIOGRAPHY 


of life from famine and disease followed the flood. During 
one of its floods, that of 1902, the Yellow River shifted its 
course so that it emptied into the Gulf of Pechili, 300 miles 
north of its former mouth in the Yellow Sea. 

The most disastrous flood that ever occurred in the United 
States was that of the spring of 1927. The water spread 
over some 20,000 square miles of the lower Mississippi 
Valley, driving about 700,000 people from their homes and 
causing a property loss of about $400,000,000. 1 

The Red Cross and the forces under Secretary Hoover 
handled the rescue of the homeless people so efficiently 
that the loss of life was much smaller than that of many 
local floods. 

Floods and Agriculture. — Flat land bordering a stream 
may be made more fertile by a deposit of silt left by a flood; 
it may be made less fertile when the flood covers the land with 
gravel and bowlders, or it may be washed away by the 
stream. 

Protection from Overflow. — The disastrous effects of 
floods may be prevented by building artificial levees or 
dikes along the banks. This has been done along a large 
part of the lower Mississippi; and the lower Rhine has not 
only been confined within high banks, but its course has been 
“ corrected,” or straightened. 

Very extensive dikes have been built along the Po River by 
the Italian Government. A line of master dikes, intended 
to confine the river during the highest floods, is built on each 
side of the river for long distances, and between them in 
many places are secondary dikes which confine the river 
during all except the highest stages of water. More than 
1000 miles of such dikes have been built along the Po and its 
tributaries. 

On the lower Mississippi levees are built of flood-plain 
materials, the largest being about 40 feet high and 200 feet 
wide at the base. In times of danger the height of the levee 

7 Extract figures not yet available. 


Fig. 2005. — Arkansas River Flood Stage at Little Rock 


RIVERS 


399 









400 


NEW PHYSIOGRAPHY 


may be temporarily raised by bags of earth. The levees 
are built alongside the river where the banks are convex; 
but where the banks are concave, the levee is built farther 
back because of the cutting and caving of the banks on this 
side. 

When the water is critically high, state guards patrol the 
levees to watch for leaks and to prevent tampering with 
the levees. Steamboats are required to keep as far away as 
possible from the levees, lest the waves generated by them 
cause the levees to break. The greatest natural enemies of 
the levees are the crayfish and the muskrat. 

This treatment will prevent floods if the levees are suffi¬ 
ciently high and strong; but it also prevents the annual 
contribution to the fertility of the soil which the floods bring, 
and there are regions where the inhabitants prefer to let the 
floods spread over the flood plains. In such localities build¬ 
ings are located on the higher lands. Loss of life will be pre¬ 
vented in a large measure if the inhabitants are warned of 
the danger of the flood. The United States Weather Bureau 
is devoting special attention to this subject and is able to 
give people warning of the approach of a flood and to tell 
them the probable stage of the water. 

A second method is to maintain outlets to distribute floods 
as quickly as possible. On the east side of the Mississippi 
just below Baton Rouge, an outlet could be maintained into 
Lake Pontchartrain by way of Bayou Manchac, a former 
distributary. On the west side of the river it would be pos¬ 
sible to maintain one through Atchafalaya, another through 
Bayou Plaquemine, and one through Bayou La Fourche. 

Because the flood plain of the Mississippi is highest near 
the river, small streams starting near the main stream flow 
away from it toward the back swamp. When a break occurs 
in the levee, much of the water of the river will soon flow 
through the opening eroded, destroying plantations. Arti¬ 
ficial openings would have to be built so that no erosion could 
occur. 


RIVERS 


401 


A third method of controlling floods is to pond the head, 
waters of the stream; i.e., to build dams across the river 
valley at places near the source of the stream. Thus lakes 
of considerable size will be formed to hold the water of the 
heavy rains and melting snows of the spring for use during 
the low water of the summer. This will prevent the spring 
flood and raise the level of the water in the dry season, thus 
making the flow of the 
stream more uniform. This 
method costs more than 
the others, but it is prob¬ 
ably more efficient and 
may render unnecessary 
the suspension of naviga¬ 
tion during the summer. 

Work of Rivers. — Dur¬ 
ing and after a rain, muddy 
water flows over the land 
and in the streams. Where 
does the mud in the stream 
come from? What effect 
does this action have on 
the general level of the 
land? How is the depth 
of the sea or lake affected 
in which the mud settles? 

The answers to the above 
questions are obvious and 
will give you a broad idea of the life work of the river. The 
work of streams may be summarized in two sentences: 
Streams drain away the surplus rainfall. In so doing they 
wear down the land and transport, comminute, and finally 
deposit the waste so formed. 

Drainage. — Our rivers remove the equivalent of about 
10 inches of rainfall per year from the whole United States. 
The Mississippi annually carries to the sea about one-ninth 





402 


NEW PHYSIOGRAPHY 


of the rainfall of the whole country, an estimated total of 
44.7 cubic miles. This is enough to make a lake the size of 
the state of Illinois, and four feet deep. 

The economic value of this water is such that the national 
government is seeking to determine the best methods of 
storing and conserving it for irrigation and power purposes. 
Government officials have estimated that the streams of the 
Southern Appalachians alone have 1,400,000 undeveloped 
horse power, worth, at $20 each, $28,000,000 per year. If 
water power, often called white coal, could be used instead of 
coal for power purposes, our diminishing coal deposits would 
be conserved. 

By adopting the third method of flood control and doling 
out the stored water during dry seasons, we not only tend 
to maintain navigation of the river, but we also increase the 
available water power during the period when it is at a 
minimum, and thus may enable mills depending upon this 
water power to continue operations. 


Destructive Processes 

Stream Erosion. — The process of wearing away the land by 
running water is known as stream erosion or corrasion. • It 
excavates the valleys occupied by the streams and is there¬ 
fore considered a destructive process in contrast with the con¬ 
structive processes that build certain features in river valleys. 

Stream erosion is seen in the gullies worn in the soil by 
a single shower (Figure 202) and in the great river valleys 
and canyons (Figures 134, 280, 299) formed by many cen¬ 
turies of erosion. In each case the land available for agri¬ 
culture in the region is diminished. The total damage to the 
farm lands of the United States through this action is 
estimated at many thousand dollars per day. 

Gullies. — A gully is a channel worn in the mantle rock 
or bedrock by running water. In Figure 202 we see that the 
soil was washed out forming a depression and that further 



RIVERS 403 

damage was done by covering other fertile land with the 
subsoil washed out. 

The damage caused by this action may be prevented, or at 
least checked, by filling the gully with brush, or by planting 
bushes or trees along the sides of the gully. Another method 
is to build water-tight walls across the gully at frequent inter¬ 
vals, forming a series of pools with falls that discharge the 
water of one pool into the one below it. 


Fig. 202. — Gully and Alluvial Cone Formed in a Single Shower 
Near Baraboo, Wisconsin. Note coarse stones in gully. Photo by Eliot Blackwelder. 

Much may be accomplished by plowing horizontal furrows 
above the upper ends of a series of gullies so as to concentrate 
the sheet drainage from above in a single gully. In this 
country, where land is so cheap, little has been done to pre¬ 
vent the growth of gullies, but in parts of Europe and Asia 
the loss is almost entirely checked. 

Stream erosion loosens the particles of soil and consolidated 
rock by solution and by abrasion. 




404 


NEW PHYSIOGRAPHY 


Solution. — Certain minerals found in the rocks are dis¬ 
solved by cold water, others by hot water, and still others 
by water containing certain gases in solution. 

When a rock contains a considerable amount of a soluble 
mineral, its solution will loosen, or tend to loosen, the in¬ 
soluble minerals. For example, a sandstone having a cal¬ 
careous cement would be converted into a sand bed by the 
solution of the cement by water containing carbon dioxide. 

Abrasion. — Pure water has little power to abrade rock, 
but water carrying particles of rock waste in suspension, or 
rolling stones along the bottom of the stream, is very effec¬ 
tive. This process is called abrasion. The agents causing it 
are friction and the blows struck by solids moved by the 
water. 

Comminution of Load. — Streams push and roll angular 
fragments of rock along their beds and over one another, 
colliding as they go, until their corners are knocked off so that 
they are rounded and worn smooth. In this way large angu¬ 
lar stones become small pebbles, characteristically smooth 
and rounded; just as boys’ marbles may be made by placing 
small pieces of marble in a cylinder, the rotation of which 
causes the pieces to wear one another round. 

Transportation. — Running water carries away (1) all ma¬ 
terial held in solution, (2) floating material, (3) the small 
particles of sand and clay that make the water muddy, and 
(4) stones and bowlders that can be rolled along the bottom 
of the river. 

Suspension — Most of the solid matter transported by a 
river is heavier than water and will sink to the bottom unless 
buoyed up by upward-moving currents in the water. Such 
currents are produced wherever the bottom of a stream is 
uneven as shown in Figure 203. These particles are said to 
be carried in suspension. This is the most important method 
of transportation by streams. The size of the largest particles 
that a stream can pick up and hold in suspension rapidly in¬ 
creases as the velocity of the stream increases. 


RIVERS 


405 


Rolling. — The size of the largest stone that can he rolled 
along the bottom depends upon the velocity of the stream. Moun¬ 
tain torrents often move stones that weigh many tons, but 
the total amount of rock waste rolled along the bottom is 
only a small fraction of that carried in suspension. 



Fig.' 203. — Section of a Stream Showing Ascending Currents 


Floating. — At certain times of the years considerable rock 
waste and driftwood, even whole trees, are carried by floating 
ice. In temperate latitudes, when the rivers break up in the 
spring, or when the 11 anchor ice ” runs in the fall, we see 
muddy ice with stones and even bowlders on and within it. 
But the total amount so carried in a year is much less than 
that carried by any other process. 

Floating matter tends to move toward shore when a stream 
is rising, for then the water surface in the middle of the 
channel is highest. But when a flood is subsiding, the water 
surface is concave, and the drift tends to leave the banks and 
to seek the middle of the channel. Lumbermen take ad¬ 
vantage of this in floating out their logs. 

Solution. — The quantity of mineral matter carried in so¬ 
lution by all the rivers of the earth is about one-third as much 
as is carried in suspension. (See Chapter XVII, page 313.) 

The total amount of rock waste carried in suspension by a 
given stream depends upon the volume and velocity of the 
stream and upon the total amount of sediment that it re¬ 
ceives from all sources. 

The Mississippi River removes yearly, by rolling along its 
bed, enough waste to cover a square mile to a depth of 19 











406 


NEW PHYSIOGRAPHY 


feet; in suspension waste, enough for 241 feet more; in solu¬ 
tion, 50 feet more if it were all limestone — a total of 310 feet. 
This is enough waste to lower the level of the whole Missis¬ 
sippi River Basin at the rate of 1 foot in about 4000 years. 

The Po removes enough waste to lower its whole basin at 
the rate of 1 foot in every 729 years. 

This great quantity of soil and rock removed includes 
much partially decomposed material that adds to the fer¬ 
tility of the soil and is a distinct loss to the farmers. 

Forests, even when hilly or mountainous, are protected 
against this loss, in a measure, by the fallen leaves that ac¬ 
cumulate and by the roots of the trees. When such regions 
are stripped of their trees, the roots no longer hold the soil, 
and the leaves that previously held much rain water will dis¬ 
appear so that gullies and steep ravines are quickly formed, 
and the soil is washed away, exposing the unfertile sub¬ 
soil. Figure 157 shows this effect. 

Certain portions of Spain and China have been rendered 
useless as farm land by the destruction of the forests. In 
our own country, in spite of the efforts of our forestry service 
to prevent it, a great deal of damage has been done in some 
of our older states. 


Features Due to Erosion of Bed 

Canyons. — In its early stages the action of stream ero¬ 
sion on consolidated rock is quite like that of a saw cutting 
through a plank. If Figure 204 represents a cross-section of 
a depression through which a stream begins to flow with the 
water collecting at the lowest point and filling the valley to 
the level BC, it is evident that erosion would be limited to 
the stream bed below BC and that the bedrock would be 
worn away only between these points. If the action were 
long continued and there were no other agents acting on the 
rock, we should have a vertical-sided valley like Figure 205 
which looks quite like a saw cut. 


RIVERS 


407 


If the depth is great in proportion to the width, these 
vertical-sided valleys are called gorges or chasms in eastern 
United States and dells or canyons in the west. They do not 
retain their precipitous side walls permanently, because the 
agents of weathering attack the side walls as soon as they 
are exposed. Figures 206 and 280 are views of canyons. 

The F-Shaped Valley. — If the stream flows through 
mantle rock, gravity will cause the side walls to slide down 
about as fast as the stream deepens the valley, giving it a 
cross-section like the letter V as in Figure 299. This is also 
the shape that the canyon must eventually assume, although 
it may take many thousand years to bring about the change. 



Fig. 204. — Cross-Section op a Fig. 205. — Cross-Section op a 


Canyon Shallow Valley 

When a stream flows over consolidated rock, the valley 
may retain its vertical sides for many centuries because 
certain rocks, like quartzite, are very slightly affected by 
weathering; but there is no rock so durable that it is not at 
all affected by weathering and that can keep its canyon 
walls permanently vertical. Canyons are temporary forms. 

Features Due to Headward Erosion 

Growth of Streams. — Stream erosion deepens stream val¬ 
leys and lengthens the main stream and all of its tributaries 
at their sources, thus increasing the area of the stream basin. 
This action is sometimes called headward erosion , but it is 
not a new process; it is merely one of the results of stream 
erosion. 

Figure 207 is a longitudinal section of the gully shown in Figure 
202. The surface of the land before the shower is indicated by the 




408 


NEW PHYSIOGRAPHY 





Fia. 206. — The Ausable Chasm, New York 
Photo by Ewing Galloway. 




RIVERS 


409 


heavy line HBCD. The portion eroded by the shower is the shaded 
area ECF. The material washed out of the gully is the dotted area 
GBE and is called an alluvial fan. The gully ends at F. Suppose 



G 8 

Fig. 207. — Showing How a Stream Valley Is Lengthened by Erosion 

the next storm cuts the valley down to the line HI. This action 
would lengthen the valley from F to I. 

All eroding streams lengthen their courses by this process. 
Some river valleys began their existence as gullies and have 
grown to their present size through stream erosion. 

Stream Piracy. — Rivers that are growing longer by ero¬ 
sion sometimes enlarge their basins at the expense of neigh¬ 
boring streams. A stream located as A, Figure 208, might in 
time drain the lake near it. To do so it would have to open 
a lower outlet than the one 
previously used by the lake. 

This would lower the level of 
the water in the lake and leave 
the former outlet above the 
level of the water. 

Another condition which favors 
stream piracy is illustrated in 
Figure 209. The line AB represents 
the top of a steep slope such as the 
front of a plateau. Stream 1 would 
have a swift current and great 
eroding power; but stream 2, flowing 
on the top of the plateau, would be sluggish. Stream 1 would, there¬ 
fore, increase its length and erode its bed more rapidly than stream 
2; and after tapping one of the branches of stream 2 would erode 




Fig. 208. — A Stream Captures a 
Lake by Headward Erosion 





410 


NEW PHYSIOGRAPHY 




Figs. 209-210. — Stream Piracy Due to Difference in Velocity 


the captured stream’s bed, reversing its flow and appropriating a sec¬ 
tion of stream 2 as shown in Figure 210. The former tributaries of 
stream 2 flow into their new master stream in the wrong direction. 



Figs. 211-212. — Stream Piracy Due to Difference in Volume 


An illustration of stream piracy due to difference in volume is 
found in the Shenandoah Valley. In Figure 211 P represents the 










RIVERS 


411 


Potomac River, and B is Beaver Dam Creek. Layers of hard rock 
crossed their paths as shown. The greater volume of the Potomac 
enabled it to erode the rock more rapidly than Beaver Dam Creek 
and soon its “Gap” in the ridge of hard rock was deeper than that 
of the creek. This gave the Shenandoah, a tributary of the Poto¬ 
mac, marked S, a steep valley and caused it to grow longer until it 
beheaded Beaver Dam Creek as shown in Figure 212. 

Erosion of Bank (Side Cutting). — Under exceptional con¬ 
ditions certain regions have been quickly drained, as when 
the wall of ice that once was the northern shore of former 
Lake Agassiz, melted 
and drained the lake. 

Streams that develop 
on such lake or sea bot¬ 
toms are called conse¬ 
quent streams , because 
their course is deter¬ 
mined by the irregular¬ 
ities of the new land. If 
the new land is smooth, 
the consequent stream 
will follow a nearly 
straight course like that 
of the Red River of the 
North (Figure 213) v 

No stream can forever 
retain its straight course, 
because erosion is constantly lowering its bed and decreasing 
the slope of the stream so that the current becomes slower 
and slower as time goes on. Therefore the stream may be 
deflected from its course by feebler forces. 

A tree falling into a sluggish stream, Figure 214, even though its 
course were perfectly straight, would deflect the stream as shown 
by the arrows in Figure 215, striking the opposite bank at some point 
as E, raising the level of the water and increasing its velocity to 
compensate for the diminished velocity at W. 



Fig. 213. — The Nearly Straight Course 
of the Red River of the North 






412 


NEW PHYSIOGRAPHY 


The inertia of the water causes it to bear hard against the bank 
from E to F and G, cutting away and undermining it until a curved 


Fig. 214. — Straight Stream with Little Erosion of Bank 


F 



X Y 


Fig. 215. — Development of Curves in a Stream 


section has been removed which starts the water across the river 
again toward the -point Y where the cutting action is repeated, as 
it is at many other places below it. The river’s course is no longer 
straight. 

This action is known as erosion of bank. It cuts away the 
land like a saw, but it is a horizontal saw that makes a curved 
bank. 


Features Due to Erosion of Bank 

The Undercut Bank. — On the outside of bends in rivers 
there is usually a vertical wall; and if the wall is formed of 



Fig. 216. — Cross-Section of a Stream Showing Erosion of the Right Bank 


mantle rock, it caves in at frequent intervals, as shown in 
the cross-section of a stream, Figure 216. 





















































RIVERS 


413 


In this sketch, WL is the water surface, somewhat higher at L 
than at W; and 1-4, 2-4, and 3-4 are successive positions of the 
vertical bank. The sketch also shows the greatest depth of the 
river at F on the outside of the bend. 

The swiftest current is also at this point. On our shallow 
rivers that may shift their channels over night, like the 
Mississippi and its tributaries, steamboat pilots make use 
of the fact that the deepest water is on the outside of a 
bend next to the undercut bank, in selecting a course for 
their boat. 

When a river is at flood stage, cutting on the outside of 
curves is most active. Sometimes strips hundreds of feet 
wide are cut away and much tillable land is destroyed. 
This action may be prevented by “ revetting ” the bank. 

The bank is graded to about a 30° slope. “ Fascines ” — 
or bundles of brush somewhat less than a foot in 
diameter and from eight to twelve feet long bound with 
willow withes — are made into “ mattresses ” consisting of 
six fascines, and the sloping bank is covered 
with the mattresses just as we shingle a house, 
with each row laid with the butts of the 
brush downstream and overlapping the row 
next below it. 

Meanders. — These are winding curves 
developed by streams as a result of erosion on the 
outside of the bends and deposition of silt on the 
inside. 

They are more apt to develop in a sluggish 
stream than in a swift one because the current 
is deflected more easily in a sluggish stream. 

When a stream begins to meander, it grows 
more and more crooked as time passes, as 
shown in Figure 217, in which the continuous lines indicate 
the original course of the stream. It is evident that, if the 
stream cuts away the bank at A and at B and deposits silt at 
C and D, the new course will be more crooked than the 



Fig. 217.— 
Growth of a 
Meander 


414 


NEW PHYSIOGRAPHY 


original. The Mississippi River is so crooked between the 
mouth of the Ohio River and New Orleans that the steam¬ 
boat trip is some four hundred miles longer than the air-line 
distance between the two places. 

Cut-offs. — Sometimes a stream straightens its course by 
eroding its bank as when a meander has developed the form 
shown in Figure 219. This stream is cutting its banks on the 
outside of each curve; and as there are two cutting banks 



Fig. 218. — A Meandering Stream 

Laramie Creek, Wyoming. Notice the nearest meander, a steep, high bank on the right 
and a low, sloping bank on the left. 


near each at X , the land between them will eventually be 
worn away, cutting off the former meander. 

Occasionally cut-offs are formed by erosion of bed during 
floods. When the stream overflows its banks and fills the 
whole valley from bluff to bluff, it flows straight down the 
valley and sometimes erodes cut-offs like A in Figure 220. 

Wide Valleys. — Erosion of bank is the principal agent in 
the formation of our wide flat-bottomed river valleys. Fig¬ 
ure 221 is a block diagram of a stream that is just beginning 
to meander in a valley bordered on each side by high bluffs. 
The dotted line shows the course that the current would take. 
It shows how erosion on the outside of the meanders, aided 




RIVERS 


415 


by weathering and gravity, will cut away the bluffs at A and 
B, making the valley wider at these points. As the meanders 



CHANNEL 


FILLING 


V 


CUTTING 


Fig. 219. — A Cut-Off Formed by Erosion of Bank 
The cutting banks are shaded. 


change their location, other parts of the bluffs are worn away 
and eventually the whole valley is widened. By this action, 
the Mississippi River has eroded the bluffs that once bordered 


Fig. 220. — A Cut-Off Formed at 
Flood Stage 



Fig. 221. — Widening a Valley by 
Bank Erosion 


it until they are more than sixty miles apart near Greenville, 
Mississippi. This is the principal method by which canyons 












416 


NEW PHYSIOGRAPHY 


and F-shaped valleys are changed to meandering rivers with 
broad and nearly level valley flats. 

Rate of Stream Erosion. — The rate of erosion varies 
greatly in different rivers. A large river, other things being 
equal, wears away much more material than a small one; 
a swift stream erodes its bed more rapidly than a slow river, 
and a river flowing through mantle rock or other easily eroded 
material will deepen its valley much faster than a similar 
river flowing through resistant rock. 

The same differences are noticed in the rates of erosion of a 
stream at a given time: (1) every change in the volume of a 
stream, as by the addition of the waters of a tributary, 
(2) every change in the character of the material forming the 
bottom of the stream, and (3) every change in the slope of 
the bed causes a corresponding change in the rate of erosion. 
Even the amount of sediment carried by the stream affects 
the rate of erosion. If the water is clear, as when a stream 
flows from a lake, the rock over which it flows is usually 
covered with moss, showing that erosion is at a standstill. 
On the other hand, if the stream receives more sediment than 
it can carry away, the stream is gradually filled so that much, 

, or all, of the water flows underground. In such a case erosion 
is practically at a standstill; but with just the right amount 
of sediment, the maximum rate is reached. 


Features Due to Variation in the Rate of Erosion 

Narrows or Water Gaps. — Rivers like the Delaware, the 
Susquehanna, the Potomac, and the James, whose courses 
are crossed by layers of very durable bedrock, are naturally 
narrow at these points. Figure 222 shows a water gap in the 
Susquehanna near Harrisburg, Pennsylvania, where the river 
has cut through Second Mountain. Consulting Figure 295, 
you will find that the upstream side of Second Mountain 
is a sandstone that stands some 800 feet above the less 
durable rocks on either side of it. The river is only about 


Plate II. Contour Map. The Appalachian Ridges near Harrisburg, Pa 

From U. S. Geol. Survey. Scale: 1 inch = 1 mile. Contour interval, 20 feet. 



































RIVERS 


417 


half as wide at the narrows at Second Mountain as it is a 
mile above. 

These narrow places are called water gaps or narrows. 
Lines of transportation usually converge toward these passes 



through the mountains, and towns frequently develop there. 
Note that two railroads, a canal, and two wagon roads pass 
through the Second Mountain water gap. 



Rapids. — Many streams have places where the current 
is so swift as to make it difficult or impossible to propel a 







418 


NEW PHYSIOGRAPHY 


boat upstream. Such places are called rapids. The swift 
current may be due to the steepness of the land over which 
a consequent stream took its course, or it may be due to a 
slope developed by the stream itself because of difference 
in the rates of erosion of two kinds of rock in the stream bed. 
Figure 223 represents limestone and shale in contact along 
thelineH'C". ABCD is the original surface on which a stream 
is supposed to develop. Since the shale is eroded faster than 
the limestone, a new bed of the stream will develop in time 
along some such line as A'B'C'D', and rapids will be found 



Fig. 225. — Structure of a Fall, that Migrates Upstream 


between B' and C. If the strata dip downward toward the 
mouth of the stream, the rapids formed will be quite per¬ 
manent; but if they dip downward toward the source of the 
stream, like those in Figures 224 and 225, or if they are hori¬ 
zontal, the rapids will soon develop into a waterfall unless 
the stream is a small one. 

Waterfalls. — The typical waterfall is one in which the 
water falls vertically as it does at Niagara (Figure 229). 
Between this and the typical rapids, there are all degrees of 
slopes which might well be called cascades (Figure 226). 
Falls are due, as a rule, to differences in the resistance of the 
rocks forming the bed of the stream. Some, like Niagara, 






















RIVERS 


419 



St. Anthony, and the Great Falls of the Missouri River, are 
due to a cap of resistant rock that overlies a mass of weaker 
rock as shown in Figures 224 and 225. 


Fig. 226 — Kepler’s Cascade in Yellowstone National Park 
Copyright J. E. Haynes. 

Others, like the two falls of the Yellowstone, and the one 
at Paterson, New Jersey, are due to dikes of lava that offer 
much greater resistance to erosion than the decomposing 
rocks above and below the dike. Still others, like the falls of 




420 


NEW PHYSIOGRAPHY 


the San Joaquin River in the Sierra Nevada Mountains, are 
due to the partial filling of a valley by a lava flow, on top of 
which the river flows and at the end of which it forms a water¬ 
fall. Occasionally falls are found at or near the mouth of a 
tributary stream, because some accident has occurred so re¬ 
cently that the tributary has not been able to adjust the level 
of its mouth to that of the master stream. The Yosemite 

Falls, shown in the 
frontispiece and in Fig¬ 
ure 135, are of this type. 
The particular accident 
that occurred to Yosem¬ 
ite Creek was the fill¬ 
ing of its former valley 
with glacial drift. The 
stream formed a new 
course and plunged over 
the very top of the wall 
of Yosemite Valley with 
a vertical drop of 1400 
feet. Figure 135 shows 
the little valley that the 
creek has cut since the 
accident. Valleys of trib¬ 
utaries like that of Yo¬ 
semite Creek are called 
hanging valleys. There 
are many falls that flow out of hanging valleys into the 
Finger Lakes, New York, like the one at the mouth of Wat¬ 
kins Glen, all of them due to lowering the levels of the lakes 
that occurred toward the close of the glacial period. 

Along our Atlantic coast a number of rivers originating on 
the eastern slope of the Appalachian Mountains and flowing 
over the front of the Piedmont Plateau form falls or rapids. 
This results in a series of falls distributed along the front 
of the plateau (Figure 227). Rapids and falls are only 


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Fig. 227. — Map op the Fall Line 














RIVERS 421 

temporary features. Both disappear before the stream 
reaches the graded stage. 

Migration of Certain Falls. — Many falls, like Niagara, 
are situated at the upstream end of a gorge of considerable 
length which has been formed by the recession of the fall. 
It has been shown by careful surveys that the center of the 
Horseshoe Fall at Niagara is traveling toward Lake Erie at 



the rate of about five feet a year. Similar, though less 
rapid recession takes place in all falls of this structure. 

In Figure 228 the line AB is the top of the wall of the Niagara 
gorge, EE is the level of the water in the gorge below the falls, and 
CD is the rocky bed of the river at the foot of the falls. As shown 
in the cross-section, inertia carries the water away from the face 
of the cliff, which is wet only by spray. Above the falls the lime¬ 
stone of the river bed is moss covered, because the river carries no 
sediment. Stream erosion there is very slow, and on the face of the 
cliff weathering acts without aid from stream erosion. How, then, 
does the cliff recede? When the falling water strikes the bedrock 























































422 


NEW PHYSIOGRAPHY 


CD, it forms two vertical “whirlpools,” J and K. These keep in 
motion all rock fragments that fall into the water, the left-hand 
whirl, J, dashing then against the face of the cliff with great force, 
cutting away the soft shale and undermining the limestone cap. 
The limestone projects more and more as the process continues 
until it finally falls into the pool below and supplies more tools with 
which to undermine the cliff. 

Some Important Falls. — At Niagara Falls (Figure 229), 
the outlet of Lake Erie plunges over a precipice 160 feet high 
on its way to Lake Ontario. Goat Island divides the stream, 
making two falls; the larger, on the Canadian side, is called 
from its shape the Horseshoe Fall; the smaller, the American 
Fall, enters the side of the gorge. The enormous volume of 
water passing over this fall gives Niagara its grandeur and 
impressiveness and makes it one of the wonders of the world. 

The upper 60 feet of the face of the fall is a hard limestone, 
in nearly horizontal layers; below this is a hardened mud or 
shale with occasional thin bedded limestones., which is very 
easily corraded. At the foot of the Horseshoe Fall the water 
is some 200 feet deep, the soft rocks at the base being worn 
away to this depth by the force with which the water strikes 
it and by bowlders which the water whirls around. Below 
the falls the river follows a gorge some seven miles long. 
Only a small portion of the water of the Niagara River is 
diverted from the falls for power purposes. 

The Genesee Falls. — The Genesee River flows over the 
same rock formations as the Niagara; but the volume of the 
water is less, and we have here three separate falls, each of 
which has at its crest a hard limestone or sandstone and 
beneath this an easily eroded shale. Below the falls the 
river flows through a gorge similar to that at Niagara, but 
narrower. 

St. Anthony’s Falls. — The Mississippi River at Minne¬ 
apolis, Minnesota, flows over a precipice capped by a some¬ 
what thinner layer of limestone than that at Niagara; and 
as the volume of water is large and the cap rock was not 


RIVERS 


423 







i 


Fig. 229. — Niagara Falls 









424 


NEW PHYSIOGRAPHY 


resistant enough to preserve the fall, it was therefore neces¬ 
sary to build a wall of cement underneath the Falls of St. An¬ 
thony in order to preserve the falls and their valuable water 
power. Here again the river, below the falls, flows through 
a gorge several miles long. 

Both falls and rapids interfere with navigation, but the 
great value of their water power leads engineers to try to 



Fig. 230. — Lock in St Mary’s Canal 


prevent their destruction and to build locks and canals so 
that vessels may pass the falls or rapids safely. Figure 230 
shows a lock built in the St. Mary’s Canal that enables ves¬ 
sels to pass around the rapids known as St. Mary’s Falls that 
formerly prevented them from passing from Lake Superior 
to Lake Huron. 

Water Power. — Water under pressure may produce me¬ 
chanical motion and is a source of power. This fact is illus¬ 
trated by the domestic water motor. There are two types 






RIVERS 


425 


of modern water wheel: one of them, known as the Pelton 
wheel, is designed to use little water under great pressure; 
the other, known as the turbine wheel, is designed for a large 
quantity of water under little pressure. 

The horse power developed by a water wheel is propor¬ 
tional to the product of the weight of the water passing 
through the wheel and the distance that the water falls in 
going from the surface of the river above the wheel to the 
level of the tailrace below the wheel. If water wheels had 
an efficiency of 100 per cent, for example, 100 pounds of 
water falling 1000 feet would develop 100,000 foot-pounds of 
energy, and an equal amount of energy would be developed 
by 10,000 pounds of water falling 10 feet. The necessary 
“ head,” as the distance the water falls is called, is easily 
obtained at waterfalls by leading the water from the stream 
above the falls through a large pipe or “ penstock ” to the 
wheel situated at the foot of the falls. 

Satisfactory conditions for the development of water 
power may be obtained on any stream with a steep slope by 
building a dam across the stream, thus forming an artificial 
fall and providing a reservoir that will store water for use 
during a dry season. A dam across the Mississippi at 
Keokuk, Iowa, enables a company to develop 300,000 elec¬ 
trical horse power which is transmitted to St. Louis, 144 
miles away; and a dam across the Susquehanna develops 
200,000 horse power by this method. 

Because of their steep slope, water-power plants are chiefly 
found in young rivers or in the young section of the large 
rivers. In the upper 70 miles of the Arkansas River, with 
its average slope of 55 feet per mile, one ton of water per 
minute could develop 3.4 horse power per mile; whereas in 
the lower thousand miles with its average slope of 0.22 foot 
per mile, the same weight of water per minute could develop 
only 0.013 horse power per mile. 

Navigable rivers cannot be obstructed by a dam without 
securing permission of the government. Sometimes a canal 


426 


NEW PHYSIOGRAPHY 


is built near the river that will carry water from a point 
upstream where the level of the water is enough higher than 
that of the stream at the point selected for the water wheel, to 
give the required head. The plan is followed at the Lachine 
Rapids on the St. Lawrence River, where the electric power 
of Montreal is produced, and at both the American and the 
Canadian power plants at Niagara Falls. Electricity de¬ 
veloped by water power is transmitted to Los Angeles, Cali¬ 
fornia, from Big Creek, 240 miles away, where the wheels 
work under a head of 2090 feet. San Francisco receives 
its power from the Sierra Nevada Mountains at Colgate, 
California. 

Important falls in the basin of the Columbia River, lo¬ 
cated at Spokane, Oregon City, the Dalles, and the Cas¬ 
cades, supply power to neighboring cities. 

Great Falls, Montana, is one of a number of important 
falls on the eastern slope of the Rocky Mountains. 

Notable falls that have played an important part in the 
development of the textile industry of New England and also 
in the growth of the cities near them are located at Fall 
River, Fitchburg, Lawrence, Lowell, and Taunton, Massa¬ 
chusetts; at Manchester, New Hampshire; at Lewiston, 
Maine; and Pawtucket and Woonsocket, Rhode Island. 
Along the fall line the following cities have valuable water 
power: Passaic,New Jersey; Philadelphia (Schuylkill River), 
Baltimore, Washington, D. C.; Richmond, Virginia; Ra¬ 
leigh, North Carolina; Camden, South Carolina; Columbia, 
South Carolina; and Augusta, Georgia. 

In New York State water power is obtained from three 
groups of streams: (1) Streams flowing into Lake Ontario 
provide power at Niagara Falls, Rochester, Auburn, Oswego, 
and Watertown; (2) the Hudson and its tributaries at 
Troy, Glens Falls, and Cohoes; (3) rivers of southern New 
York at Jamestown, Binghamton, and Elmira. 

Valuable water power is obtained at Minneapolis, Min¬ 
nesota, from the falls at Sault Ste Marie, Michigan, and from 


RIVERS 


427 

a number of large rapids in various parts of the country, 
notably those of the St. Lawrence River. 

Constructive Processes 

Deposition. If one fills a bottle with muddy water, pref¬ 
erably taken from a swift stream, he will notice that as soon 
as the water is quiet, coarse sand begins to settle to the bot¬ 
tom of the bottle. This sand will be followed by finer par¬ 
ticles, and after a day or so the water will become clear and 
colorless. 

Deposition occurs in all quiet water and in all running water , 
whenever the velocity of the water is decreased. 

When a stream flows into quiet water, such as a lake or the 
sea, all of the sediment brought in by the stream is deposited, 
forming deltas, or continental shelves, or filling lakes. 

Deposition in the bed of a river occurs when a tributary flows 
into a master stream with a gentler slope. The amount thus 
deposited is usually greater than the master stream can carry 
away, and the deposit that accumulates may be an alluvial 
fan; or a dam across the master stream, such as was formed 
in the Mississippi by the Chippewa, and by the Colorado 
River in the Gulf of California. 

Deposition in the bed of a river may be caused by a decrease 
in the velocity of the stream , so that sand bars are formed in 
the channel; or it may form natural levees bordering the chan¬ 
nel, or flood plains on one or both sides of the channel. 

The principal cause of decreases in the velocity of a stream 
is a decrease of slope; but velocity may also be decreased with¬ 
out a change in slope, by decrease in volume , such as occurs 
in some of the rivers flowing eastward from the Rocky Moun¬ 
tains. Some of these rivers leave the mountains bank-full of 
water and loaded with rock waste. As the water seeps into 
the dry earth of the plain, more and more of its load is de¬ 
posited until, as sometimes happens, the stream disappears, 
leaving all of its sediment in the channel. Velocity is also 
decreased when a stream spreads out in a wider and shallower 


428 


NEW PHYSIOGRAPHY 


sheet, as it does when it leaves its channel and flows over a 
flood plain. This is one of the causes of deposition on the 
flood plain. 

Local causes, such as obstructions or the deflection of 
the current to one side of the stream, cause deposition on the 
downstream side of the obstruction and on the side of a 
stream opposite to a cutting bank. 

Assorting. — Careful examination of the deposits formed 
in the bottle of muddy water will show that the material is 
assorted; a layer of coarse sediment at the bottom with layers 
of finer and finer particles above it. 

In quiet water this happens because fine particles settle 
more slowly than coarser ones. 

If a stream bed has several slopes as shown in Figure 231, 
with a decrease of slope of the stream bed at B' and another 
at C', it will be found that a deposit of particles of quite 
uniform size is accumulating at each point, and furthermore 
that the particles accumulating at B' are distinctly larger 


A 



Longitudinal section of a stream. ABCD is the water surface, 
and A’B'CD’ the bed of the stream. 

than those at C'. This is because the size of the particles 
that can be transported depends upon the velocity of the 
stream. At B' only the particles too large to be carried by 
the new velocity will be deposited; and similarly, a deposit 
of the coarsest particles carried beyond B' will be found at C'. 

The assorting action due to local causes is well shown in Fig¬ 
ure 232. The arrows show the direction of the current. At AB a 
projecting point is seen. Below this point is a deposit of sand made 





Fig. 232. — Stream Deposits Due to Local Causes 


RIVERS 


429 












430 


NEW PHYSIOGRAPHY 


when the water was higher, in the quiet water below the point. In 
the center at DD' are large bowlders that the water could not carry 
away. At F is the gravel that deposited below the point E, and 
at H is a mass of cobblestones and coarse gravel that accumulated 
below the log GG'. Can you name the deposits at C, D, F, and H 
in the order of the swiftness of the currents that deposited them? 
On the downstream side of the log GG', the deposit has filled the 
stream bed nearly to the top of the log. 

Engineers often build obstructions running out from the 
shore of a river to cause filling of this kind. These obstruc- 


A 



Fig. 233. — Stratification of River Deposits Due to Decreasing Velocity 


tions help to reclaim portions of the river bed, or they narrow 
the river, thus increasing the velocity and scouring out a 
deeper channel. Some of the dams built for this purpose in 
western rivers have caused deposits similar to flood plains, 
having areas of several hundred acres; and when these de¬ 
posits are forested, they become quite permanent. The jet¬ 
ties of the lower Mississippi River, by narrowing the river, 
cause it to scour out a deep channel. 

Island sand bars sometimes form very quickly just below 
an obstruction, such as a sunken boat or a lodged snag. Some 
years ago a barge sank in the Wisconsin River. In a few 
days an island of sand several times as large as the barge had 
formed below. 

Stratification. — As long as the velocity and the volume of 
a stream remain constant, the character of a deposit at a 
given place will remain the same. But both velocity and 




RIVERS 


431 


volume are subject to great changes. The size of the par¬ 
ticles deposited at a given place, therefore, changes from time 
to time, forming layers of sediment that differ in coarseness 
as shown in Figure 233. This process is called stratification. 

All river deposits are assorted and stratified, but neither 
the assorting action nor the stratification is so perfect as the 
similar action performed by ocean waves; and the deposits 
made by ocean waves cover vastly larger areas than those 
formed by rivers. 


Features in a River Valley Due to Deposition 


Sand Bars. — When the velocity of a stream is decreased 
because of widening of the river bed, sand bars sometimes 
form that extend across the wide part of the river and that 
have a steep slope on the downstream side and a gentle orie 
on the Upstream side. These bars migrate in the direction of 
the current, just as a sand dune does, and for the same reason. 
The current rolls sand grains up the gentle slope; and they 
pour over the crest of the bar, constantly building up a new 


A 



B 

D 


Fig. 234. — Sand Bar Formed Where the River Widens 


downstream slope which stands at the angle of repose of sand 
under water. 

The crest of such bars often comes nearly to the surface of 
the water in the stream, in which case the location of the bar 
is indicated by a ripple on the surface that warns the steam¬ 
boat men of the shallow water. 

Figure 234 is a longitudinal section of a stream with such 
a sand bar. The arrows show the direction of the current and 
of the motion of the sand grains up the gentle slope of the bar. 

Overloaded Streams. — In some rivers the load of rock 
waste is so great that the stream cannot move it appreciably 







432 


NEW PHYSIOGRAPHY 


except at flood stage. At low water most of the flow is 
through the sand as ground water, with only an occasional 
surface channel. These channels interlacing produce a 
braided stream. Most of the tributaries of the Mississippi 
on the west are overloaded. 

Flood Plains. — When lateral erosion has produced a val¬ 
ley that is wider than is required to carry away the water of 



Fig. 235. — Cross-Section of a Flood Plain 


the stream at ordinary stages, the flow of water is confined 
to the deepest portion of the valley, called the channel, leav¬ 
ing wide areas on either side of the channel that are valuable 
farm lands. These are called flood 'plains, because they are 
subject to inundation during floods. (See Figure 235.) 

When water covers a flood plain, friction retards the ve¬ 
locity of the shallow water, causing a deposit to accumulate 
on its surface that gradually raises the surface higher and 

higher and adds to the fertility 
of the land. The surface of 
the deposits is smoothed by the 
running water, and the flood plain 
as a whole acquires a downstream 
slope that is approximately paral¬ 
lel to water surface of the stream. 



PIG - v 6 ShTpST i ST 8 " . Figure 235 is a cross-section of a 

river valley. The line ABC may be 
assumed to be the bedrock not removed when the river was 
eroding its bed. Above that surface are layers of gravel, sand, and 
mud that have accumulated since erosion of bed ceased. The de¬ 
pression between L and L' is the low-water channel. The high- 
water level is indicated by the dotted line. 

















RIVERS 


433 


In some instances young streams with F-shaped valleys 
and swift currents have been overloaded by their tributaries 
and their valleys filled with coarse sediment forming flats that 
are much less fertile than the true flood plain (Figure 236). 

Natural Levees.—The greatest loss of velocity as a river 
spreads over its flood plain occurs where the water leaves 



the channel, that is to say, along the edges of the channel; 
and here the coarsest and the largest deposit is formed. 
The margin of the low-water channel is thus raised faster 
than the rest of the flood plain forming ridges LL', Fig¬ 
ure 235, that are called natural levees. 

The natural levees are usually high and dry and follow 
the meanders of the channel. They provide the best loca¬ 
tion for home sites and roads on the flood plain. 








434 


NEW PHYSIOGRAPHY 


Along the outer margin of the flood plain, where the river 
deposits join the old land, the surface is generally lower than 
at points nearer the channel and is often swampy. Back 
swamps are common features of flood plains. 

Deltas. — At the mouth of a river the velocity of the water 
is checked, and the sediments carried by the stream are de- 



Fig. 238. — Delta of the Nile 


posited, forming a delta. At first, the delta appears above 
the water as mud banks along the margins of the sev¬ 
eral channels or distributaries (Figure 237). These banks 
are really extensions of the natural levees. The space be¬ 
tween the distributaries is gradually filled in and becomes an 
extension of the flood plain of which the delta is really a 
part. 

The delta of the Nile River, Figure 238, shows many dis¬ 
tributaries with the space between them filled with deposited 
material. Figure 237 shows the delta of the Mississippi in an 






RIVERS 


435 



earlier stage. The deposits have not everywhere accumulated 
so as to form land. The “ natural-levee ” portions, however, 
are all above water and border the distributaries, here called 
passes. The straight lines extending the levees at the South 
Pass are the “ jetties.” 

Deltas are apt to form at the mouths of all rivers that 


Fig. 239. — Alluvial Cone at the Mouth op Aztec Gulch, Colorado 
Photo by U. S. G. S. 

empty into a body of water not agitated by strong waves and 
shore currents, such as a protected bay or sea. The deltas 
of the Mississippi, the Po, the Nile, the Bramaputra, the 
Danube, and the Yukon, illustrate this condition. The 
St. Lawrence, the Susquehanna, and the Amazon have im¬ 
perfect deltas because they empty into the open ocean where 
waves and currents spread the deposits along the shore and 
also because the shores are sinking. 

Alluvial Fans and Cones. — When a stream leaves a valley 
having a steep slope and flows over level land like a piedmont 
plain or a flood plain, the load of rock waste that it was 
carrying in the steep section is deposited in semicircular form 





436 


NEW PHYSIOGRAPHY 


as in Figures 239 and 240. If the deposit is nearly flat, it is 
called an alluvial fan; but if it rises steeply, it is called an 
alluvial cone. 

River Profiles. — A river profile is a line showing the changes 
in slope of a river, using definite vertical and horizontal scales. 
We frequently use an eighth or a tenth of an inch to represent 
a mile horizontally, whereas the same distance vertically will 



Fig. 240. — Alluvial Cone with Tributary and Distributary Streams 
Note contours on cone. 


represent only ten feet. Profiles are usually drawn on paper 
ruled especially for the purpose. 

In Figure 241, the figures across the bottom show the number of 
miles from the mouth, and those on the sides give the elevation of 
points of the profiles in feet. 

In the Columbia-Snake profile the river bed on either side of the 
point marked 3 is convex to the sky, as also is the bed at 5. At 2, 
4, and 6, however, the bed is concave to the sky. 

The profile of the Arkansas River shows that the bed is concave 
to the sky at all points. It is much nearer the profile of equilibrium 
than the Columbia-Snake River. 










RIVERS 


437 


The Profile of Equilibrium. — The processes of erosion and 
deposition will eventually change the profile of a river from 
the irregular form of the Columbia-Snake to the concave^ 



The large squares represent 100 miles along their horizontal sides, 
but only 1000 feet along their vertical sides. 


Feet 

7000 

6000 

5000 

4000 

3000 

2000 

1000 

0 


form of the Arkansas. Erosion, acting alone, tends to wear 
off the high points, and deposition alone tends to fill the 
hollows. 

If the deposits at B' and C', Figure 231, continue long enough, 
they will produce a uniform slope from A' to D'. In Figure 242, 
ABCD represents a portion of the profile of a river. If erosion wears 
away the point B and deposits cover the point C, the profile will 
take the form AB'C'D; and in time the profile between A and D 
will be the straight line AD. 


Deposition tends to fill places that are concave to the sky, and 
erosion tends to wear away places that are convex to the sky. 

When various slopes of a stream are so adjusted that each 
section can just carry the load that it receives, the stream is said 


Columbia 

















































438 


NEW PHYSIOGRAPHY 


to have the profile of equilibrium , and the bed of the river is 
said to be graded. 

Summary. — The present characteristics of a river seem 
to depend less upon the conditions of its origin than upon the 
processes of erosion and deposition that have modified the 
river in the following ways: 

1. The length has increased by headward erosion (page 407) and 
by seaward extension at the mouth through growth of a delta (page 
434) or through uplift of the shore. 

2. The velocity of the current diminishes as erosion lowers the 



stream bed, thus diminishing the total fall from the point lowered, 
to the sea. This also diminishes the rate of erosion of bed. 

3. Erosion of bed ceases at a given point when deposition begins 
there, or when the section of the stream is eroded to sea level. 

4. Crookedness increases as the velocity of the stream decreases 
(page 412). 

5. The valley grows wider (page 414). 

6. The profiles are graded , causing the disappearance of falls and 
rapids and the draining or filling of lakes (pages 437, 438). 

7. The number of tributaries increases for a time, and then di¬ 
minishes. 

8. The divides between the streams are lowered and adjusted. 

Normal Development of a River. — The changes just men¬ 
tioned lead to slow but definite development of rivers as 
time elapses and give certain characteristics to the region 
drained by the river; it is therefore quite easy to recognize 
the following stages in the normal cycle of development: 
youth, the beginning of the life work of the stream; maturity , 





RIVERS 


439 


the period following the first rapid changes, in which the 
stream has acquired many permanent characteristics; old 
age, the period in which completion of the life work is ap¬ 
proached. The record of these stages is the life history of the 
river. 

Characteristics of a Young Region. — Several characteris¬ 
tics of young regions are shown in Figures 202, 206, and 280. 

Examine these pictures and note which of the following charac¬ 
teristics are illustrated in each: (1) The stream’s course is rela¬ 
tively straight. (2) The slope is irregular. (3) The profile is 
convex upward in places. (4) The valley is F-shaped or a canyon. 
(5) There are few tributaries. (6) There are falls, rapids, and lakes 
in the region. (7) The average slope of the bed is steeper than it 
will ever be again. (8) The current is correspondingly swifter. 

(9) The most important work of a young stream is erosion of bed. 

(10) Interstream areas are flat, and the divides or lines that separate 
the basins of adjoining streams are poorly defined. (11) The drain¬ 
age of the region is poor, as shown by the presence of swamps, ponds, 
and lakes. 

Influence of Youthful Rivers on Man. — The lakes in 
young rivers tend to diminish the danger of floods and to 
make the flow of the stream greater during dry weather. 
They also provide water supply in many cases. 

Falls and rapids furnish water power. If there are deep 
valleys, storage reservoirs for irrigation and power purposes 
may be easily constructed. 

Navigation is interrupted by falls and rapids, and cities 
are usually located near them. Louisville, Kentucky, near 
the rapids of the Ohio River, and Minneapolis, Minnesota, 
near the Falls of St. Anthony, are examples. 

The flat interstream areas are easily tilled. Farms are 
located on them, as are the railroads, wagon roads, and town 
sites of the region. 

Characteristics of a Mature Region. — (1) The course is 
crooked. (2) The profile is everywhere concave to the sky. 


440 


NEW PHYSIOGRAPHY 


(3) The slope of the water surface is more uniform. (4) The 
slope is less steep than in youth. (5) The current is cor¬ 
respondingly slower. (6) The valley is deep with wide bot¬ 
tom lands called flood plains. (7) Water gaps, passes, and 
meanders have developed. (8) Tributaries are very nu¬ 
merous. (9) Falls and rapids have disappeared. (10) Inter¬ 
stream areas are steep slopes with well-defined divides. 
(11) The drainage of the region is good as shown by the dis- 



Fig. 243. — The Flood Plain of the Canadian River, Oklahoma 


appearance of lakes and swamps. (12) The principal kinds 
of work performed are erosion of bank and transportation. 

Influence of Mature Topography on Man. — Agriculture 
is mostly confined to the flood plains, though it has to con¬ 
tend with spring floods there. Navigation of. the river is 
interfered with by sand bars and the low water of the summer 
months. Towns, wagon roads, and railroads are, as a rule, 
located in the stream valleys, and flood plains are the only 
areas for town sites or for farming. Mining is the most 
important industry of the interstream spaces in many mature 
regions, because of the ease with which we may discover 




RIVERS 


441 

valuable minerals if any are in the layers of bedrock exposed 
in the numerous valleys of the region. Other possible indus¬ 
tries are, forestry, hunting, and trapping; but in the absence 
of mining the inhabitants of these regions usually live a life 
of poverty, hardship, and ignorance. 

Economic Importance of Flood Plains. — The soil of flood 
plains is rich in plant food and is easily tilled. The large 
proportion of silt in the deposits makes the capillary distribu¬ 
tion of the ground water well-nigh perfect; and since the 
water table is usually near the surface, flood plains rarely 
suffer from drought. The neighboring river provides an 
easily traveled highway, which makes flood plains excep¬ 
tionally accessible. These two characteristics, fertile soil and 
accessibility, have made flood plains so desirable for settle¬ 
ment that they are nearly everywhere densely populated. 

The flood plain of the Mississippi below the mouth of the 
Ohio is from 20 to 50 miles wide and about 600 miles long. 
The region is densely populated,, and fine crops of corn, cot¬ 
ton, and sugar-cane are raised there. 

The flood plain of the lower Rhine is one of the most 
densely populated and carefully cultivated regions of Europe. 
The flood plain of the Yellow River, in China, probably has 
a denser population than any other region in the world. 

The advantage which the less strenuous struggle for exist¬ 
ence gave the ancient inhabitants of flood plains over the in¬ 
habitants of less favored regions, is shown in history. Egypt 
developed on the flood plain of the Nile, and Chaldea and 
Babylon on the plains of the Euphrates and the Tigris. 
These nations were so important among the ancients that 
the period prior to 800 b.c. is sometimes mentioned as the 
“ fluvial period ” of history. 

Characteristics of an Old Region. — Figure 275 shows 
an old region. (1) The most important work of an old 
river is transportation and deposition. Erosion has prac¬ 
tically ceased. (2) The slope is very gentle. (3) The cur¬ 
rent is sluggish. (4) The course is very crooked. (5) The 


442 


NEW PHYSIOGRAPHY 


profile is nearly horizontal. (6) The valley is shallow, and 
steep slopes are absent both from the stream bed and the 
valley sides. (7) Tributaries are fewer, many having been 
lost by migration of divides. (8) Interstream areas are 
again flat. (9) Drainage of the region is good except on the 
flood plains, because elsewhere the land slopes gently toward 
the streams. (10) Oxbow lakes and natural levees have 
developed. 

Influence of Old Topography on Man. — The area adapted 
to agriculture is greater in an old region than in any previous 
stage, because both flood plains and the interstream areas 
are adapted to it. Portions of the flood plains may be wet 
and require draining, but the fertility of the land when 
drained will amply repay the farmer for his labor. Because 
of the absence of steep slopes, towns, wagon roads, and rail¬ 
roads may be located wherever they are desired. There are 
no falls or rapids to interfere with navigation of an old stream; 
but sand bars are apt to form in the channel, and the shift¬ 
ing of the channel is a constant annoyance to steamboat 
men. Sometimes a channel shifts so far away from a town 
which developed on the banks of the river, that river traffic 
there is no longer important. 

The Interrupted Cycle. — Many streams never complete 
their normal cycle because of accidents which interrupt their 
development. The principal causes that interfere with a 
stream’s development are change of slope due to depression 
or elevation of the stream bed and change of climate that 
affects the rainfall. 

Effects of Depression. — If depression occurs at the mouth 
of a river, the sea will enter the lower portions of the river 
valleys, drowning them and producing hays, estuaries, or 
fiords. Tributaries near the mouth of a river enter bays, and 
the master stream is said to be dismembered. The lower 
Susquehanna, with its former tributaries, the James, the 
York, and the Potomac, was drowned and dismembered, 
forming Chesapeake Bay, with its many branches. 


RIVERS 


443 


The general effect of depression is to shorten the cycle. This 
is because depression decreases the work that the river may 
accomplish. 

Effects of Elevation. — Elevation of a region at the mouth 
of a river lengthens the river. When two or more rivers thus 
lengthened unite, they form an engrafted river. Rivers may 
also be engrafted by the extension of their deltas into the 
same bay. The tributaries of the Mississippi River below 
Cairo have been engrafted upon it. 

If elevation takes place at the source, the slope is increased, 
and the river is rejuvenated. A meandering river, if reju¬ 
venated, forms entrenched meanders. Rivers entrenching 
themselves in flood plains sometimes leave portions of the 
old flood plain persisting as alluvial terraces. 

If the uplift is across its course , the river may corrade 
downward as rapidly as the uplift is made, thus producing 
water gaps. 

The Green River passes through the Uinta Mountains 
and the Hudson through the Highlands. Because such 
rivers had their approximate location before the mountains 
were uplifted, they are called antecedent rivers. 


Lakes 

Characteristics of Lakes. — Ponds and lakes are relatively 
large bodies of water, filling basins or depressions in the 
earth’s surface to the lowest point in their rims, at which point 
the excess of water flows away forming the lake’s outlet. 

The water of the lake is without appreciable current; 
hence sediments brought in by streams will be deposited, 
leaving the water clear and apparently pure. This is notably 
the case with the waters flowing from large lakes like Erie 
and Ontario and also with the water of the Rhone where it 
emerges from Lake Geneva. 

Functions of Lakes. — The chief functions of the lake are: 
(1) as a reservoir of water; (2) as a regulator, maintaining 


444 NEW PHYSIOGRAPHY 

a more uniform flow below the lake; (3) as a settling basin 
for sediments; (4) as an equalizer of the temperature of 
bordering lands. 

Lakes are essentially great reservoirs of water. If large, 
they clarify the muddy water of inflowing streams, so that 
rivers that are the outlets of lakes are generally clear. Even 
the greatest rain does not raise the level of large lakes very 
much, so that their outlets are not subject to great floods. 
It takes a long time to lower the flood level of the lake. It is 
for this same reason that in times of drought outlets of lakes 
vary in volume less than other rivers. The Great Lakes thus 
regulate the flow in the St. Lawrence River; but the neigh¬ 
boring Ohio River, without any lake, is subject to great 
floods. Large lakes ameliorate the climate of their vicinity, 
particularly on their lee side; this is because water changes 
its temperature slowly, making the lakes in summer cooler, 
and in winter warmer, than the neighboring land. 

Sources of Lake Water. — Lakes receive water from three 
sources: (1) inflowing streams, (2) ground water, (3) rain 
that falls directly into the lake. The indirect source of all 
of this water is atmospheric precipitation. The divide or 
watershed of a river or a lake is the ridge or line surrounding 
the area from which water flows into the river or lake. 

Origin of Lake Basins. — Lake basins originate in many 
ways: 

1. Movement of the Earth’s Crust. — (a) Some lake basins, 
those of Lake Superior and the Caspian Sea, for example, 
are believed to be the result of uplift of a portion of the bottom 
of the former sea that cut off and enclosed a portion of the sea. 
Some of the early myths and legends of the Greeks seem to 
indicate a former passage through the Black and Caspian 
Seas to the Arctic Ocean. 

(b) Other lake basins are believed to be due to the depression 
of their basins. A large but shallow basin of this type, 
known as Reelfoot Lake, was formed in the Mississippi valley 
by an earthquake in 1811. Other examples of this type 


RIVERS 


445 


are Lake Baikal, more than a mile deep, and the lakes of the 
Great Rift Valley extending from the Sea of Galilee through 
the Dead and Red Seas into the lake region of Africa. 

2. Basins Formed by Volcanoes. -— The more common kind 
of volcanic lake basin is the crater of an inactive volcano. 
Crater Lake, Oregon, Figure 244, and Lake Avernus near 
Naples, Italy, are fine examples. These basins are usually 
circular with high, precipitous banks, and the water in them 
is often very deep. In southern Germany are older crater 
lakes, with low, gently sloping banks. Crater Lake is about 
five miles in diameter and 2000 feet deep. It is surrounded 
by steep walls that rise 2200 feet above the water in places. 
It is believed that the former volcano caved in, forming the 
lake basin. 

Another type of lake basin due to volcanic activity is 
found at the base of certain volcanoes. For example, Mt. 
Shasta, California, has several of them which were made by 
intersecting lava flows. Such basins are very irregular in 
shape. 

3. Glacial Basins. — Four kinds of basins owe their 

existence to glacial action (page 478): (a) rock basins; 

(6) ancient river valleys partly filled by drift , like the basins 
of the Finger Lakes of New York State; (c) kettle holes 
(irregular depressions in glacial deposit; become lakes if 
the bottom of the depression is clay); ( d ) marginal lake 
basins. When a glacier blocks the natural drainage of a 
region, the water accumulates in the basin and finally over¬ 
flows at the lowest point in the divide that incloses the 
basin, forming a marginal lake with a wall of ice for its 
shore, on one side. Some of the largest lake basins ever 
formed were of this kind, but most of them have disappeared. 
Former Lakes Duluth, Chicago, and Maumee, in Figure 268, 
Part 1, and Lakes Algonquin and Iroquois in Part 3 of 
Figure 268 were marginal lakes. 

4. Coastal Plain Basins. — When the first rainfall de¬ 
velops a stream on an uplifted continental shelf, it fills all 


446 


NEW PHYSIOGRAPHY 


depressions in its path to the level of its lowest outlet. 
Many lakes formed in this way are still in evidence on the 
Atlantic coastal plain. 

The four types of lake basins described are found in young 
regions, but basins formed by rivers occur in mature and in 
old regions. 

5. Lake Basins Formed by Rivers. — 

(a) Oxbow lakes. Figure 245 is a map of the flood plain of the 
Mississippi River near the mouth of Big Black River. It shows 
five oxbow lakes, lettered A, B, C, D, and E. Lake A shows an 
early stage in the formation of an oxbow basin. A cut-off has been 

formed recently, but the lake is 
still connected with the cut-off at 
both ends, and the water of the 
river still comes to the little town 
just east of the lake. Lakes C and 
D have been separated from the 
river at one end by a deposit of 
sand. At the other end, water 
connection with the river is still 
possible. Lakes B and E are en¬ 
tirely disconnected from the river 
by sand deposited in the ends of 
the former meander. 

The three stages in the forma¬ 
tion of an oxbow lake basin are: 
the formation of a meander, the 
formation of a cut-off (lake A), the separation of the basin from 
the river channel by deposition (lakes B and E). 

( b) Basins at the Mouths of Tributaries. Rivers that carry much 
sediment sometimes build up their natural levees faster than the 
tributary can fill the lower part of its valley. The levee thus be¬ 
comes a dam that ponds the water of the tributary until enough ac¬ 
cumulates to flow over the top of the levee, thus forming a lake. 

The Red River of Louisiana has a number of lakes where tribu¬ 
taries reach its flood plain. 

(c) Basins Formed by Deposits of Tributary Streams. The Chip¬ 
pewa River, because of its steep slope, deposits more sediment in 



Fig. 245. — Oxbow Lakes, Sand De¬ 
posits, and Main Channel of the 
Mississippi Riveb 



Fig. 244 . — Crater Lake, Oregon 

Showing the “Phantom Ship” and the high walls. Courtesy of A. M. Prentice, Portland, Oregon. 


RIVERS 


447 














448 


NEW PHYSIOGRAPHY 


the Mississippi River than the latter can carry away, thus forming 
an obstruction that ponds the water of the Mississippi above the 
obstruction. 


Reference Table of Principal Lakes 


Name 

Abea in 
Sq. Mi. 

Altitude 
in Ft. 

Maxi¬ 

mum 

Depth 

Comparisons 

Caspian. 

170,000 

- 85 

3,200 

California 

158,000 

Superior. 

31,200 

602 

1,008 

South Carolina 

30,000 

Victoria Nyanza.. 

26,000 

800 

240 

West Virginia 

25,000 

Aral. 

25,050 

160 

1,200 



Michigan. 

22,500 

581 

870 



Huron. 

22,320 

581 

700 



Nyassa. 

14,200 

1,500 

2,300 

Maryland 

12,000 

Baikal. 

13,000 

1,700 

5,600 



Tanganyika. 

12,000 

2,700 

2,100 



Great Bear. 

11,200 

390 

270 



Erie. 

9,960 

573 

200 

New Hampshire 

9,300 

Winnipeg. 

9,400 

710 

70 

New Hampshire 

9,300 

Ontario. 

7,240 

247 

738 

New Jersey 

7,800 

Chad (dry season) 

6,000 

900 

8 



Chad (wet season) 

40,000 


20 



Titicaca. 

3,200 

12,500 

700 

Delaware 

2,000 

Nicaragua. 

2,800 


.... 



Great Salt Lake. . 

2,200 


50 



Champlain. 

480 


400 

New York City 

327 

Dead Sea. 

360 

- 1,268 

1,300 



Chelan. 

85 

1,079 

1,500 



Como. 

60 

650 

1,340 



Crater. 

25 

6,239 

2,000 




Salt Lakes. — Certain lakes like Lake Champlain and 
Salton Sea began their existence as bodies of salt water 
connected with the ocean. 

The relation between the volume of water that flows into a lake 
and the volume lost by seepage and evaporation determines 
whether a lake remains salt or is gradually freshened. 

Figure 246 is a cross-section of a lake basin filled with salt 
water to the level A A. The lowest point in the rim of the 
basin is at 0. 

If the volume of the inflow of water is less than the volume 
lost, the level of the lake will be gradually lowered; but as the 






























RIVERS 


449 



Fig. 246. — The Formation op Salt Lakes 


level is lowered, the loss is diminished, because the areas of 
both evaporation and seepage are decreased so that when a 
certain level, as BB, is reached, the loss will equal the inflow. 
After this the lake will maintain about the same level, hut will 
grow more and more salty as time goes on, because of the min¬ 
erals brought in (Chap¬ 
ter XVII, page 313). -———- 

This is the history of 
the Caspian Sea and of 
Salton Sea. 

If, on the other hand, 
the inflow of water ex¬ 
ceeds the losses, the 

water level will rise. When the level of the outlet 00, is 
reached, the excess will run off, and the lake will become fresh. 
This is what happened to Lake Champlain. 

Certain lakes began as fresh-water lakes, but by a change 
of climate the inflow was decreased until it was equal to the 
loss. They then became salt and gradually grew more and 
more salty just as Salton Sea did. This is the history of 
Great Salt Lake. 

Alkaline Lakes. — Either Glauber’s salt, soda, borax, or 
Chili saltpeter exceeds the quantity of common salt in the 
waters of certain lakes and gives the water a bitter taste. 
Such lakes are known as alkaline lakes. 

White deposits of some of these chemicals may be seen on 
the shores and upon all objects that project above the water 
of Soda Lake (Figure 247). Searle’s lake, or marsh, contains 
all of these chemicals, and a number of lakes in the Great 


Basin are important sources of one or more of them. 

Destruction of Lakes. — Lakes are temporary features in 
rivers; even the oxbow and delta lakes will eventually disap¬ 
pear, although traditions seem to indicate that some of them 
have not been materially changed since the white man first 
saw them. 

Many lakes have been drained by erosion of the outlet 


j 


t 






450 


NEW PHYSIOGRAPHY 


channel; in time all lakes whose bottoms are above sea level 
must disappear through this action, unless some other agent 
destroys them first. 

A second method of destroying lakes is by filling. Some 
five miles of the southern end of Seneca Lake, New York, 
have been filled by sediment brought in by streams; and 



Fig. 247. — Soda Lake near Parma, California 
Photo by Siebenthal, U. S. G. S. 


deltas are forming in nearly all lakes where streams enter, 
diminishing their size. Figure 248 is a picture of such a 
delta that has nearly divided the lake. Lake St. Clair, 
between Lakes Huron and Erie, has been greatly diminished 
in a similar way. If sufficient time were allowed, this cause 
alone would also destroy all lakes. 

Some lakes are filled with vegetable matter. The lake in 
Figure 249 shows an early stage of this process. Certain 



RIVERS 


451 


kinds of moss and some other plants sometimes grow on the 
surface and hold wind-blown sand and dust which gradually 
spreads over the lake, forming a floating bog. A railroad 
line in Minnesota crossed such a bog. Cattle grazed upon it 
before the line was built; but the engineers discovered that 
the floating bog was a mass of vegetable matter and dust 
four feet thick and that beneath it was twenty feet of water. 



Fig. 248. — Delta Built into a Lake 
Silvaplana, Switzerland. 


Prairie Tremblant , Louisiana, is a floating bog through 
holes in which fish may be caught. Eel grass and wild rice 
also assist in filling many lakes. 

Marl deposits, formed largely of animal remains which 
form in some lakes to the depth of many feet, also assist in 
filling lakes. 

These methods of filling gradually convert a lake into a 
swamp or marsh, and many of our fresh-water marshes are 
former lakes destroyed in this way. The student will 





452 NEW PHYSIOGRAPHY 

doubtless be able to find examples of such marshes near his 
home. 

A third method of destroying lakes is by evaporation. The 
great Lake Bonneville, that once covered a part of the 
Great Basin as large as Lake Huron, was partially destroyed 
by evaporation. Its supply of rain water was cut off by a 
change in climate, and the lake shrank gradually, until 
to-day Great Salt Lake is all that remains. 


Fig. 249. — How Vegetation Destroys a Lake 

Economic Importance of Lakes. — The lake is the natural 
reservoir for the storage of water, and water storage has 
become so important that man constructs basins of many 
sizes and for many purposes. 

There are the outdoor swimming pools, the mill ponds, the 
great reservoirs for city water, and still larger ones to supply 
water for irrigation or to control the flow of water in rivers. 

Concerning reservoirs for city water supply Mr. Merriam, 
Chief Engineer of the New York Board of Water Supply, 
gives the following information: 






RIVERS 


453 


New York City has provided an available storage capacity 
of about 149,000,000,000 in two reservoirs, the Ashokan 
and the Schoharie in the Cat skill Mountains. In addition, 
three other reservoirs were constructed: namely, Kensico, 
Hill View and Silver Lake. The cost of the entire Catskill 
system, including the aqueduct to the city, is about 
$188,000,000. 

At the present rate of increase, the city’s available water 
resources may suffice to about the year 1935. No decision 
has as yet been reached regarding future additional sources. 

The Roosevelt Dam, shown in Figure 194, forms a basin 
covering more than 16,000 acres and more than 250 feet 
deep and supplies water to the inhabitants of the Salt River 
district of Arizona. 

The Great Lakes are important arteries of commerce as 
also were the smaller lakes before the advent of the railroad. 

These smaller lakes are of great value as pleasure and 
health resorts. 


QUESTIONS 

1. Compare the effects of elevation and of depression on the 
length of the river cycle, with examples. 

2. Do the same for a change of climate from moist to arid, 
from arid to moist. 

3. In what ways, and with what results, may a normal river 
cycle be interrupted? 

4. Account for meanders. 

5. Why may a river valley be in some portions young and in 
other portions mature or old? 

6. What would be the effect on Lakes Erie and Ontario if 
eastern Canada should be slowly uplifted? 

7. Distinguish an estuary and a delta. 

8. Why are falls and rapids merely incidents in the life history 
of a river? 

9. Discuss the advantages and disadvantages of falls. 

10. Discuss: “Rivers are the mortal enemies of lakes.” 

11. How did fresh water Lake Bonneville change to Great 
Salt Lake? 


454 


NEW PHYSIOGRAPHY 


12. How can the steamer, Figure 230, get through the lock to 
the lower level in the foreground? 

13. Distinguish between the constructive and the destructive 
work of rivgrs. 

14. Why do certain falls retreat upstream? Where will such 
a fall cease to exist? Study figure 225. 

15. Why do divides shift? 

16. What kinds of lake basins are made by streams? 

17. What kinds of lake basins are made by other agents besides 
streams? 

18. Name several kinds of lake basins that were formed before 
the stream existed. 

19. Name three kinds of lake basins that may be formed after 
a stream has begun its work. 

20. In what respects does the Mississippi River resemble a tree? 

21. What is arborescent drainage? 


i 


CHAPTER XXI 


GLACIERS 

Introduction. — In almost every part of the United States 
snow sometimes falls. In the southern lowlands it may last 
but for a few hours or days; whereas in the mountains and 
in the northern section of the country it may remain for 
months, or even throughout the year. Mount Washington 
has snow fields far into the summer, and Mount Hood is 
perpetually snow-capped. Traveling northward into Canada, 
we find more extensive snow fields reaching ever lower 
levels, until they finally reach sea level within the Arctic 
Circle. 

Origin. — Wherever more snow falls than disappears 
during the year, the excess accumulates; and by compression 
and successive freezings and thawings gradually changes to 
ice. Gravity causes the mass of snow and ice to move slowly 
toward sea level. These moving masses of ice are glaciers. 

A glacier is a mass of ice, formed from snow above the snow line y 
that moves slowly toward sea level through the action of gravity. 

The snow line, above which there is perpetual snow and 
above which glaciers originate, is about three and one-half 
miles above sea level at the equator, and descends toward 
sea level with increase of latitude northward and southward, 
reaching sea level within the polar circles. The more ex¬ 
tensive the area above the snow line the more extensive the 
glaciers. 

Two types of glaciers result: those formed in valleys among 
mountains, due to altitude, called valley or alpine glaciers; 
and those which form extensive ice sheets, in high latitudes , 
known as continental glaciers. 

455 


456 


NEW PHYSIOGRAPHY 


The Ice Front. — A valley glacier moves down to warmer 
levels, and a continental glacier moves outward from the 
center of accumulation. Each eventually reaches a limit 
where the ice melts as fast as the glacier moves. This limit 
is the ice front. 

Changes in temperature or in the rate of motion of the 
glacier may cause the ice front to advance or to retreat, 
but so long as these conditions are unchanged the ice front 
is stationary. 

The retreat of the ice front is in no sense a backward move¬ 
ment of the ice, as ice movement is always down the valley 
or slope; it means only that the rate of forward movement 
does not equal the rate of melting back, and the ice front takes 
a position farther up the slope. 

Water from the melting ice often forms ponds on the 
surface of the glacier, and short streams plunge into the 
crevasses, becoming subglacial streams. Some of these 
streams flow along beneath the ice, emerging at the ice 
front; others may flow along in the ice, emerging as spout¬ 
ing streams, high up on the ice front. 

The subglacial stream washes out the finer particles of the 
waste that it encounters, leaving the coarser particles under 
the ice. This makes the stream milky or muddy. 

Distribution. — Valley glaciers are found on every conti¬ 
nent except Australia, occurring in Africa and South America 
even under the equator; also upon some mountainous is¬ 
lands, as New Zealand. In North America, small glaciers 
occur in the United States in the Cascades, the Sierras, and 
the Rockies, increasing in extent in the Canadian Rockies 
and in Alaska. Some of these glacial regions, as the Alps, 
Canadian Rockies, and Alaska, attract many tourists on 
account of their peculiar grandeur and beauty. 

Continental glaciers occur on all land areas where the 
snow line descends to the general level of the land. Glaciers 
form only on land, and the ice which forms over the polar seas 
is not glacial ice. The most extensive continental glaciers 


GLACIERS 


457 



Fig. 250. — The Matterhorn 

Look for the moraine at the foot of the glacier. Photo by Wehrli 







458 


NEW PHYSIOGRAPHY 


are the ice sheet which covers Greenland, and that which 
covers the Antarctic continent. The Greenland ice sheet is 
about 500,000 square miles in area. That of the Antarctic 
continent is greater than the United States in area. Several 
Arctic explorers have penetrated far toward the center of the 
Greenland ice cap, and some have crossed it. In the interior 
it rises to an altitude of perhaps 10,000 feet, with a tempera- 



Fig. 251. — Mont Blanc above the Snow Line (to Right of Center) 


ture constantly below freezing, and therefore is one of the 
most absolutely desert regions of the earth. 

Movement. — From their sources in the fields of granular 
snow and ice, called neve, the valley glaciers move down the 
valleys as rivers, of ice, descending into the midst of forests 
and even into cultivated fields. They evaporate and melt 
as they move forward, becoming smaller and smaller, and 
finally disappear where the melting back just balances the 
forward movement of the ice. 

These ice rivers behave much the same as rivers of water, 




GLACIERS 


459 


eroding their beds and transporting their load of waste. 
Like ordinary rivers, too, they move faster in the middle 
than at the sides, faster at the top than at the bottom, and 
the line of swiftest flow lies nearest the convex side of a 
curve in the ice stream. The glacial river also has its rapids 
and falls, analogous to those in an ordinary river. While 
we know that glaciers move, the movement of most gla¬ 
ciers is so slow as to escape the notice of all but the most 
observant. It required careful measurements to discover 
the manner of their movement. 

The Swiss glaciers move generally only a few inches a 
day, moving faster in summer; whereas some of the Alaskan 
glaciers move as much as seven feet a day. 

Cause of Motion. — About the only thing regarding the 
method of movement upon which all are agreed is that a 
glacier does not move as a solid block of ice slipping down the 
slope. 

One explanation of its motion supposes that the glacial 
ice is granular and that the pressure above causes the grains 
to move on and over one another. This may be illustrated 
by the movement of moist brown sugar when piled up. 

Another explanation attributes the movement to alter¬ 
nate freezings and thawings. The great pressure above 
Causes the ice to melt, particle by particle. Each particle, as 
water, occupies less space; the pressure upon the melted par¬ 
ticle is decreased, and the particle freezes again at a lower 
level, only to be remelted by the pressure. The glacier move¬ 
ment is thus the sum of the movements of its grains. 

A simple experiment may be made that suggests the prob¬ 
ability of each of the above explanations: (1) Take a long 
block of ice, and rest the ends upon supports. After some 
time the block will be bent, much as a thin board supported 
in the same way, although the surrounding temperature 
is below freezing. (2) Support a block of ice by a fine wire 
around it. The wire will quickly cut through the block, the 
two pieces again freezing together below the wire, even 


460 


NEW PHYSIOGRAPHY 



Fig. 252.—The Rhone Glacier 











GLACIERS 


461 



though the temperature is above freezing. The pressure of 
the block upon the wire melts the ice, and the water thus 
formed immediately freezes again with release of pressure. 


Fig. 253. — Crevasses in the Eiger Glacier 

It is a matter of common occurrence that a block of ice rest¬ 
ing upon another soon freezes to it for like reason. 

Effect of Movement. — As a result of glacier movement, 
the snow is slowly drained away from the mountain slopes. 



462 


NEW PHYSIOGRAPHY 


Because of the unequal movements in the glacier, it becomes 
very much broken. These breaks are called crevasses. 

Where a glacier passes from any slope to a steeper slope, 
the bending and the more rapid movement causes a series 

of transverse 
crevasses to form 
across the glacier. 
They are more 
common in the 
upper courses of 
glaciers, although 
the Rhone glacier 
ends in such an ice 
rapid. If the slope 
becomes again less 
steep, these crevasses disappear by closing up and melting 
of the surface. 

Because of the more rapid movement of the glacier at the 
middle than at the sides, there develop a series of oblique 
cracks, which become ever wider as they advance down the 
slope. They are the lateral crevasses, and they make walk¬ 
ing upon the lower courses of glaciers very difficult and 
dangerous, especially when the winter snows have tempo¬ 
rarily bridged them over. Sometimes a glacier passes from 
a narrow to a wider valley; and the ice, spreading out later¬ 
ally, produces longitudinal crevasses. 

Glacial Mills. — The surfaces of most valley glaciers are 
too much broken to permit the formation of streams upon 
them from the melting ice in summer; but occasionally 
such streams are formed. These sooner or later tumble 
into a crevasse, and armed with the bowlders and finer ma¬ 
terials which also find their way there, grind depressions in 
the bedrock beneath the ice. These are glacial mills, and 
their grist is the materials which serve them as tools. Larger 
and more numerous streams form on continental glaciers, 
and the pot holes ground out beneath them are larger. 



Fig. 254. — Representation of Glacial Movement 

The strip ST changes to S'T'. Because ice cannot 
stretch, it tends to crack at right angles to the line T'i', 
so that the crevasses formed point obliquely upstream. 











GLACIERS 


463 


Work of Glaciers. — Glaciers drain away precipitation in 
the form of snow, and like rivers, erode their beds, transport 
their load of waste, and when they melt deposit it. 

1. Drainage. — An area about equal to that of the United 
States is drained by continental and valley glaciers. 

2. Erosion. — Ice, like water, has little power to erode; 
but when supplied with rock waste imbedded in its under 



Fig. 255. — Glacial Scratches (toward Us), Glacial Bowlders, and Pothole, 
Glacier Garden, Lucerne 


surface, it becomes a powerful agent of erosion. The weak 
and weathered portions of the bedrock are removed; and 
the fresh and harder portions are rounded, striated, and 
grooved, the striae and grooves being parallel to the direc¬ 
tion of movement of the ice. Such rounded masses of the 
bedrock are called roches moutonnes. 

Valley glaciers ream out their valleys, changing U-shaped 
valleys to the £/-shape. At their sources one often sees 
great amphitheaterlike basins, called cirques , on the moun- 





464 NEW PHYSIOGRAPHY 

tain side, formed by the plucking away by the snow and 
ice of the fragments of bedrock loosened by weathering. 

Continental glaciers plane down the uplands, rounding 
off the irregularities of the hills and ridges, and filling trans¬ 
verse valleys in their course. 

3. Transportation. — All glaciers carry waste. The con- 


Fig. 256 —Upper Grindelwald Glacier 

Glacial scratches proceeding from under the ice at the left side. 

Note the [/-shaped valley in the background. 

tinental glacier gets its load from the surface over which 
it moves; the valley glacier gets the greater part of its load 
from the bordering slopes. Whether carried upon the sur¬ 
face of the glacier, within the ice or beneath it, the ma¬ 
terials are known as moraine. Materials along the sides of 
a valley glacier constitute the lateral moraine; that be¬ 
neath the ice the ground moraine; and that about the end 
of the glacier the terminal moraine. When two glaciers 
join, the united lateral moraines between them, continued 



GLACIERS 


465 



Fig. 257. — Famous Rosegg Glacier 

Showing tongue of ice with crevasses, moraines, and ice-born stream. 
Note the cirque in the background. 



Fig. 258 — Cross-Section of a Glacier 
Showing lateral and ground moraine, crevasses, and ice table. Walther. 





up 15 or 20 feet above the general surface of the glacier. 
Large slabs of stone are often left perched on pedestals of 
ice by the melting of the ice around them. Such perched 
stones are known as glacial tables. 

Transportation by moving ice differs materially from trans¬ 
portation by moving water in that the size of the largest 
particle that can be transported by the water depends upon 
the velocity of the stream, whereas moving ice transports 
its load of rock waste without regard to the size of the frag¬ 
ments. Bowlders the size of a house mixed with particles 
of the finest clay are transported by the glacier. 

Glacial motion is slower than that of other agents of 
transportation, but every mass of rock that falls upon its 
surface, or that is picked up at the sides or bottom of the 
ice, will be carried away without regard to size. 


Fig. 259. — Two Views of Same Glaciated Pebble of Limestone from Chicago 
The eighteen facets indicate alternate fixity and change of stone to ice. 


466 NEW PHYSIOGRAPHY 


upon the glacier below the junction, is the medial moraine. 
If the medial moraine is abundant, it may so protect the 
ice beneath from melting that the morainic ridge may stand 


GLACIERS 


467 


Shelter Rock, Figure 122, was brought to Long Island by- 
glacial ice from the highlands of the Hudson River. Its 
exact size is unknown, because it is partially buried in the 
ground. At its highest point its top is 16 feet above the 
ground, and the exposed portion is 40 feet wide and 54 feet 
long. A rectangular block of granite of this size would 
weigh about 2500 tons. 

4. Marking. — While the materials in the ground moraine 
are subjected to the crushing weight of the glacier, the 
bowlders and pebbles in it being often polished and striated, 
the materials of the moraines are, for the greater part, not 
rounded as are those carried by rivers. 

5. Deposition. — When a glacier melts, its load of waste 
is deposited — not in layers and assorted, as is the waste 
carried by rivers, but pell-mell without trace of assorting. 

The terminal moraine marks the limit reached by the 
glacier; and since glaciers vary with the season in their 
rate of movement, a series of concentric ridges mark suc- 



Fig. 260. — Moraine Deposits at the End of Glacier, Switzerland 


cessive retreats of the glacier front. These may later be 
overrun by the glacier during a period of extension down 
the valley. 

When a continental glacier melts, the great quantity of 




468 


NEW PHYSIOGRAPHY 


water produced carries away much of the till and deposits it 
elsewhere in strata of gravel, sand, and clay. These deposits 
are indirectly due to the glacier. 

It is estimated that at one time during the melting of the 
last North American continental glacier the water from the 
melting ice made the Mississippi River sixty times as large 
as it now is. It is therefore not strange that there are many 
great deposits of assorted and stratified drift about the 



Fig. 260 A. — A Natural Cross-Section of an Esker, Mendon, New York 

southern border of the former ice sheet and along the lower 
Mississippi River. 

The indirect deposits of a glacier are kames, eskers, out- 
wash plains and deltas, and the loess deposits forming natu¬ 
ral levees along the outflowing streams from the glacial 
sheet. Kames are mounds of imperfectly stratified gravel and 
sand that accumulated in depressions along the ice front. 
When the ice walls of the depressions melted, the deposits 
became mounds. 

Eskers are imperfectly stratified deposits of coarse sand and 
gravel. They consist of particles too large to be carried 




GLACIERS 


469 




away by the subglacial stream, probably due to blocking o‘f 
its channel. Figure 260A is a photograph of a cross-section 
of an esker at Mendon, New York. The lower part of this 
esker is composed mostly of 
small bowlders six or eight 
inches in diameter; the upper 
part is of finer material. 

Figure 260 B is a view of a 
curve in an esker, suggesting 
a meander in the subglacial 
stream. 

Outwash plains are nearly 
level areas formed of strati¬ 
fied sand, gravel, and clay, 
that border the terminal 
moraine of a continental 
glacier. The Long Island outwash plain is from ten to 
twenty miles wide. Its chief characteristic is indicated by 


Fig. 260 B . — A Small Esker 


Fig. 260C. — A Delta Formed when the Water in Lake Keuka, New York, 
Was 300 Feet Higher than at Present 

such place names as Flatbush, Flatlands, and the plains of 
Hempstead. 

A similar deposit about a valley glacier is called a valley 
train . 





470 


NEW PHYSIOGRAPHY 


' 6. Deltas. — When the waste-laden streams from a glacier 
enter a lake or other body of water, they form deltas of 
glacial material which often serve as the best evidence of 
former water levels. Figure 260(7 is a side view of such a 
delta formed when the water in Lake Keuka, New York, 
stood three hundred feet higher than it does now. 

Figure 261 is a diagram of the relative locations of some 
glacial deposits to the ice front. The buried block of ice, 



Fig. 261. — Relative Positions of Some Glacial Deposits Near the Ice Front- 


when melted, would allow the moraine to fall in, forming a 
kettle hole on the terminal moraine. 

The Subglacial Stream. — From the end of every valley 
glacier, and at frequent intervals along the front of the 
continental glacier, issues a subglacial stream. These 
streams, supplied chiefly from the melting ice, are much 
stronger in summer than in winter. They are usually muddy 
from the load of rock flour they bear, derived from the ground 
moraine. 

In valley glaciers the subglacial streams usually deposit 
their load in some lake or river. Where glaciers reach the 
sea, the subglacial stream may issue beneath the sea level; 





GLACIERS 


471 



and the glacier, instead of melting back, may break off in 
blocks and float away as icebergs. 

Former Extension of Glaciers. — As we may trace the 


Fig. 262. — The End op the Grindelwald Glacier 

Occupying the bottom of a* great 17-shaped valley. Note the cirquelike cliffs at the 
base of the Viescherhorn in the background. The cliff on the right, smoothed to the 
top, indicates a much more extensive glacier here in former times. 

shore line of a vanished lake by the characteristic shore-line 
features, so we may recognize the former existence of glaciers 
in regions where now no glaciers are found. Glacial records 






472 


NEW PHYSIOGRAPHY 



263. — Extent of the Ice Sheet in North America 

R. E. Howell. 


























GLACIERS 


473 



are so characteristic as to be usually unmistakable. We 
observe the work of the glaciers now existing, and we know 
that glaciers of the past did the same sort of work. There¬ 
fore, when we find polished and striated surfaces on the 
valley sides far above the present glaciers in the Alps, we 
do not hesitate to expand our glaciers to those heights; and 
when we find Z7-shaped valleys in any region, though now 
far removed from any modern glacier, in imagination we re- 


Fig. 264. — Typical Rounding by Glacial Action 

Note bowlders deposited by the glacier. U-shaped valley in the background 
Kerguelen Island. (Penck.) 

store the glacier, for ice alone seems competent to make 
Z7-shaped valleys. 

Thus extended, the Alps become a very much larger glacial 
region, extending to the plains of northern Italy, and the 
miniature glaciers now found in the United States become 
the centers of similar regions. 

Of even more interest, and of much greater economic im¬ 
portance, is the former extension or existence of continental 
glaciers. While doing the same sort of work as valley gla¬ 
ciers, the records made by continental glaciers are more 
varied and more enduring. These records are continent 






474 


NEW PHYSIOGRAPHY 


wide, and may be read alike in the planing down of the high¬ 
lands and in the filling and leveling up of the lowlands. 
Reading the records, we discover that there was a time, in 
the not distant past as earth time is measured, when much 
of northern Europe, extending to and including the British 
Isles, and most of North America down to the latitude of 
New York City, were covered by continental ice sheets. 



Fig. 265. — Map of Erie Moraines 


This time is known as the Ice Age or Glacial Period. Many 
other parts of the earth have had glacial climates, some of 
them probably several times. 

The Ice Age in North America. — If we travel across the 
United States from north to south, we are impressed by 
the unlikeness of the topography in the north and in the 
south. 

In the north the rivers are young, often having rapids and 
falls; and lakes are numerous. The uplands are level; or, 
if uneven, the hills and ridges are covered with a cloak of 



















































GLACIERS 


475 


unassorted and usually coarse mantle rock. Numerous 
bowlders, wholly different from the bedrock of the region, 
are widely scattered, especially in the east. The mountains 
have numerous lakes and swamps in their valleys. The 
soils are all transported, being entirely unlike the decom¬ 
position products of the bedrock. 

In the south the rivers of the upland are mature, having 
long ago removed their rapids and falls. There are no lakes 
or swamps in the mountains or in the uplands; and the 
uplands are hilly and cloaked with residual soils. 

The line which separates these two types of topography 
follows roughly the Missouri and Ohio Rivers to their 
sources in the Rocky Mountains and in southwestern New 
York; thence westward by an irregular line to Puget Sound 
and eastward and southeastward through New York City 
to the east end of Long Island. This line marks the south¬ 
ern limit of the ice during the Glacial Period, and is the 
southern boundary of the deposits made by the continental 
ice sheet. 

Retreat of the Ice Sheet. — As the ice sheet moved down 
from the north, it invaded a region probably about as ma¬ 
turely dissected as Kentucky and Tennessee now are. River 
systems were widely branching, and lakes had disappeared. 
When with change of climate the ice sheet began to melt 
back, a wholly changed land surface was revealed. Ridges 
were planed down, and valleys partially or wholly filled. 
Wherever the ice sheet paused for a time in its retreat, there 
was formed a terminal moraine. If the ice advanced for a 
season, former moraines were obliterated, to be succeeded by 
new when retreat again began. The ice sheet did not ad¬ 
vance and recede equally along its entire front, and records 
of various advances and retreats remain. Many terminal 
moraines or halting places, roughly parallel, are found be¬ 
tween the Ohio River and the Great Lakes. 

In melting, the ice often left great bowlders perched in 
unstable positions. Such bowlders are often rocking stones , 


476 


NEW PHYSIOGRAPHY 


and are unquestioned work of the ice, as running water 
would not leave them thus. 

By overrunning the ground moraine, long lenticular hills 
called drumlins were fashioned. The formation of drumlins 
may be likened to the leaving of pointed strips of packed 
snow when wet snow is swept from the pavement. 



Fig. 266. — Typical Unassorted Drift near Lake Grinned 
Photo by New Jersey Geological Survey. 

The general name for all deposits left by the ice is drift; 
and while valleys parallel to the direction of the ice move¬ 
ment were often kept free of drift, or even deepened by the 
ice, valleys transverse to the direction of its movement 
were generally drift-filled. 

From the time of the advance of the ice sheet south from 
the present position of the Great Lakes, then probably a 
river valley, until its retreat north of them, all drainage 
was southward to the Gulf or eastward to the Atlantic. 




GLACIERS 


477 


The Great Lakes themselves developed outlets southward 
to the Mississippi when first freed from the ice. 

With retreat of the ice front northward from the divide 
separating the Hudson Bay and Gulf of Mexico drainage 
basins, a great lake was formed between the north-sloping 
land to the south and the ice front to the north. The outlet 
of this lake southward developed a wide, deep valley, now 
occupied by the Red River of the North. With the melting 
of the ice dam, this lake disappeared, and its silt-covered 
bed is now one of the greatest wheat-producing regions in 
North America. This glacial lake is called Agassiz. 

Further retreat of the ice sheet revealed thie more favor¬ 
able route eastward through the Mohawk Valley, and the 



Fig. 267. — A Drumlin at Ipswich, Massachusetts 


southward outflow of the Great Lakes ceased. The Mohawk 
route was in turn abandoned for the present route of the 
St. Lawrence, when the glacier had sufficiently melted. 

With the melting of the glacial sheet all rivers issuing from 
its front were swollen beyond their usual volume and beyond 
the capacity of their ordinary channels. The burden of 
rock flour carried by these rivers was thus spread along 
their banks as natural levees, forming a peculiarly fine and 
even-textured deposit known as loess. Great thicknesses of 
loess are found along the Missouri River at and below Kansas 
City and along the Mississippi southward as far as Baton 
Rouge. That it was deposited at least in part by the rivers is 
indicated by the occurrence in it at Vicksburg and elsewhere 
of numerous snail shells; also by its gradual thinning and 
final disappearance a few miles back from the river front. 






478 


NEW PHYSIOGRAPHY 


Similar deposits in Germany and China have been attributed 
in part to the wind; but the glacial origin of the material 
is undoubted. 

Lakes and Marshes. — Mention has already been made 
of the occurrence of numerous lakes in the glaciated area of 
the United States and of their absence south of this area; and 
the origin of some of the lake basins has been suggested. Lake 
Agassiz was perhaps the least common type of glacial lakes, 
although the lakes produced by temporary ice dams were per¬ 
haps the most extensive. The Great Lakes, which individ¬ 
ually were probably in basins in part produced by glacial 
drift interrupting the drainage in a preglacial valley, were for 

a time united into one 
greater lake by the ice 
sheet blocking the outlet 
through the St. 
Lawrence. While all 
were united and stood 
at a common level, they 
were separated into 
three somewhat distinct 
basins by the high land 
south of Georgian Bay, 
which rose as an island 
in the midst of their icy 
waters. To the Superior- 
Huron-Michigan divi¬ 
sion the name Algonquin has been given; greater Ontario 
has been called Iroquois; and modern Erie bears the name 
of its larger ancestor. 

The margins of these greater lakes have been traced by 
the beaches and other shore-line features then developed. 
With the melting of the ice these lakes gradually assumed 
their modern proportions. For their development see 
Figure 269. 

Other large lakes, like the Finger Lakes of New York, and 










GLACIERS 


479 



Fig. 269. — Development of Great Lakes at End of the Ice Age 

Stages indicated by outlet: in No. 1, separate, in No. 2, Lake Maumee into Lake 
Chicago only; in No. 3, through the Mohawk; in No. 4, through Ottawa. 























































480 


NEW PHYSIOGRAPHY 


most of the lakes of northern New York and New England, 
occupy basins produced by drift deposited in valleys, and 
represent the unfilled remnants of preglacial rivers. 

Westward from New York, and north of the Great Lakes, 
by far the most numerous type of glacial lakes occupy local 
depressions in the drift. As the ice sheet receded, it left an 
uneven surface, and in the depressions lakes were formed. 
These were often shallow and quickly changed to the marsh 
stage. In this way the numerous high-level marshes of the 
northern United States and Canada were formed. 

Still another type of lake was produced near the southern 
limit of the ice sheet when the glacier came down to the sea. 
In southeastern New York and along the New England 
coast, deep and sometimes circular lakes occur. These 
kettle lakes probably represent the resting place of a detached 
mass of ice over which drift was deposited. Shallower 
lakes of the same type are rush-filled, or have become dry 
lake beds. 

Economic Importance of the Drift. — The presence of the 
drift in the northern section of our country has played an 
important part in determining the lines of its economic 
development. The general result of the deposit of the 
drift was to leave this region more nearly level than before 
the coming of the glacier. This has favored the building 
of roads and railroads in the section, which in turn promoted 
commerce. 

A deeper covering of mantle rock is found in the glaciated 
than in the unglaciated regions, and this favors the more even 
and constant flow of rivers. The numerous lakes here also 
equalize the flow of streams, making transportation by water 
possible. River transportation in the south is both local 
and limited; whereas in the north our lakes, our rivers, and 
our canals make carriage by water of great and growing 
economic importance. 

Mining in the drift-covered section is of little importance, 
since the thick coat of drift has made the discovery of impor- 


GLACIERS 


481 


tant mineral deposits difficult. With the exception of oil, 
gas, coal, and salt, important mineral deposits have been 
discovered and developed only where the drift covering 
is thin or where streams have cut deep valleys. 

The soils of the two sections are very unlike, but it is 
difficult to determine whether the drift has furnished a 
better or poorer soil than would have developed from the 
bedrock beneath. In the eastern part of the drift-covered 
section the soils are too coarse and sandy, and the surface is 
cumbered'with glacial bowlders; but farther west the soils 
are fine textured, free from bowlders, and very productive. 
It is probable, however, that the difference in character of 
crops raised in the two sections is due rather to difference 
of climate than to difference of soil. 

The economic products obtained from the drift are clay, 
sand, and gravel. The clays are manufactured into bricks, 
tiles, and crockery; the sands into glass and brick; and the 
gravels are used for road making. 

Causes of the Glacial Period. — Much speculation has 
been indulged in regarding the causes of glacial periods. No 
single cause has been generally considered competent. 
Glacial conditions now exist on high mountains in all lati¬ 
tudes; in high latitutes such conditions exist even down 
to sea level. The^ three following hypotheses, briefly out¬ 
lined, are worthy of mention: 

1. It is maintained that glacial climates were produced by ele¬ 
vation of extensive land areas. The elevation of the regions east and 
west of Hudson Bay — the centers of accumulation of ice during 
the last glacial period — by only a few thousand feet would again 
make those regions glacial centers. A glacial sheet once started, 
the temperatures about its borders are made lower, and the sheet 
extends. We know there was great elevation in northern United 
States and northward in preglacial times. It would thus require 
an elevation no greater than has occurred to bring another glacial 
period to northern United States. 

2. This theory maintains that the development of glacial ice 


482 


NEW PHYSIOGRAPHY 


sheets results from increased length of winter combined with in¬ 
creased distance from the sun. At present, our winter occurs 
when we are 3,000,000 miles nearer the sun than we are in sum¬ 
mer, and moreover, our winter is about seven days shorter than 
our summer. The shape of the orbit of the earth changes, 
becoming less nearly a circle, and the position of the earth’s 
axis so changes that there comes a time when we are far¬ 
thest from the sun in winter, and our winter is longer than our sum¬ 
mer. This hypothesis makes our glacial climates the result of long 
aphelion winters. According to this hypothesis, glacial conditions 
now exist in the southern hemisphere; and the fact that the only 
extensive land area in high southern latitudes is covered by a thick 
sheet of ice seems to support this hypothesis. 

3. Changes in climate that produce glacial ice sheets and later 
melt them are largely the result of a change in the amount of carbon 
dioxide and water vapor in the air. These changes accompany 
changes in the relative areas of land and sea, due to movements 
of the earth’s crust. With increase of land comes an increase of 
carbon dioxide in the air, with warmer climate and consequent in¬ 
crease of water vapor. With decrease of land, both the carbon 
dioxide and the water vapor in the air decrease, and the climate 
grows colder. This hypothesis requires glacial climates in both 
the northern and the southern hemispheres at the same time instead 
of alternately as required by the second hypothesis. 


QUESTIONS 

1. What parts of the earth are now having glacial climates? 

2. Name regions that have been but are no longer covered by ice. 

3. Locate the regions now having great valley glaciers. 

4. Give three explanations of the movement of glaciers. 

5. Apply to the explanation of the different kinds of crevasses 
the statement: “ice breaks at right angles to direction of strain.” 

6. Tabulate a comparison and contrast of glaciers with streams 
as to movement , work, and deposits. 

7. Which side of a glaciated hill of bedrock is marked by ice 
scratches and on which side are deposits made? 

8. In what direction will an ice table finally fall from its ice 
pedestal and why? 


GLACIERS 


483 


9. Why do we find numerous lakes in Wisconsin, Indiana, and 
New England and none in Kentucky, Tennessee, and the Carolinas? 

10. Why is the land in southeastern Ohio hilly, whereas that in 
the central and northern parts of the state is level? 

11. Many New England fields are surrounded by walls of stone. 
WHiy? W r hy do we not see similar walls in Virginia? 

12. Why are there so many more streams shown on maps of 
Missouri in that part qf. the state north of the Missouri River than 
south of it? 

13. How do drumlins show the direction of movement of the ice? 

14. When and why did the Finger Lakes in New York State 
reverse their direction of outflow? 

15. The Maumee River in Ohio was once a tributary of the 
Wabash. Why is it no longer so? 

16. What is the probable source of the great granite bowlders 
found strewn over the land from Boston to Minneapolis? 

17. Explain the “hanging” valleys entering the Finger Lakes of 
New York. 

18. WLy are there more glaciers on the southwestern side of 
Mount Shasta in California than on the eastern side ? 

19. WTy is the snow line higher on the northern slopes of the 
Himalayas than on the southern slope? 

20. Explain the glacial plains that border Lake Ontario. 


CHAPTER XXII 


PLAINS 

Relief. — The highest points of the continental masses 
rise less than six miles above sea level, and the deepest parts 
of the ocean are a little more than six miles below sea level. 
The extreme departure of the lithosphere from a perfect 
spheroid is about twelve miles, which is less than one six- 
hundredth of the diameter of the earth. 

This slight irregularity, which gives us our continents and 
ocean basins, is called relief. 

The important relief features cf the earth are mountains, 
plateaus, and plains; and although their difference in height 
above sea level is often only a small fraction of the extreme 
relief of the lithosphere, yet it causes marked differences in 
their climates, their adaptability to agriculture, and in the 
habits and occupations of their inhabitants. 

Definition. — A plain is a broad, relatively smooth land that no¬ 
where appears conspicuously higher than adjoining land or water. 

Plains are usually the lowlands cf the earth, some of them 
smooth as a ballroom floor, others rolling. Their relative 
smoothness depends upon (1) the manner in which they 
were formed and (2) the amount of erosion that has 
occurred since they were formed. 

The smooth surface of many plains is due to the fact that 
the material forming the plain was deposited in water and 
that the movement of the water spread it out in layers 
parallel to the surface of the water. When this under-water 
surface emerges from the water, it becomes a plain, unless 
it is crumpled or broken in the process. 

484 


PLAINS 


485 


Economic Importance of Plains. — The comparatively 
even surface of the plain makes the building of roads, rail¬ 
roads, and canals easier than in hilly regions and also de¬ 
creases the labor of the farmer. 

The soil of the plain is likely to be finer, deeper, and more 
fertile than the adjoining higher land, because the surface 
wash of the rain on the slopes brings down to the plain the 
finer particles of soil, as well as much of the soluble plant food. 

The shallow valleys of the plain tend to keep the water 
table near the surface, thus rendering crops less likely to 
suffer from drought. The lower altitude of the plain gives 
it a higher average temperature and a somewhat longer 
growing season than the adjacent higher lands. These condi¬ 
tions favor agriculture and contribute to progress in civili¬ 
zation and the development of trade. For these reasons the 
great majority of the people of the world live on plains. 

In tropical and semitropical lands, tropic vegetation 
flourishes on the lowlands, whereas, at higher altitudes, the 
crops of the temperate zone, such as wheat and barley, can 
be grown. Again, a plain in the rain shadow of a mountain 
range may have a fertile soil, yet not be suited to agriculture 
because of deficient rainfall. Similarly, plains in the trade- 
wind belts suffer from lack of rainfall; and plains in the 
polar regions cannot be important agricultural lands be¬ 
cause of the low temperature, although in high latitudes the 
coastal lowlands are the only regions where crops of any 
kind can be raised. 

As a rule, the well-watered plains of the temperate zones 
are the great agricultural regions of the world and support 
the densest populations. 

Plains Formed by Deposition. — Many plains consist of 
deposits of rock waste that have been given a smooth, flat 
upper surface in their deposition. 

Most of the agents that transport and deposit mantle 
rock (page 274) form flat-topped deposits. The character¬ 
istics of these deposits will depend upon (1) the agent trans- 


486 


NEW PHYSIOGRAPHY 


porting the material and (2) the conditions under which it 
was deposited. The classification is based upon their origin. 

There are four classes of plains of deposition: 

1. Marine plains, formed by emergence of sea bottom. Usually 

coastal. 

2. Lake plains, formed by the destruction of lakes (page 451). 

3. River plains. 

(a) Flood plains, formed along a river course. 

( b ) Deltas, formed in some body of water at the mouth of a 
river. 

4. Glacial plains. 

(а) Till plains, left by a receding ice sheet. 

(б) Outwash plains, deposited in front of ice sheet by water. 

Coastal plains are composed of rock waste which was 
carried to the ocean by streams or was cut from sea cliffs 
by the waves. Wave action and the tidal currents spread 
the rock waste out in the smooth, gently sloping deposits 
familiar to us as beaches; but the beach is only the part 
of the deposit that is above low-water mark, and the deposit 
sometimes extends a hundred miles from shore. 

Such deposits are called continental shelves (page 227) while 
they remain submerged, but become coastal plains when they 
emerge from the sea. 

Like all aqueous deposits, those of the coastal plains are 
assorted and stratified, and the layers are nearly horizontal 
except where they have been distorted during emergence. 

The layers of gravel and sand are porous and permit 
ground water to flow through them readily, whereas the 
layers of clay and mud are more impervious. 

These conditions make possible the numerous artesian 
wells so common in coastal plains. 

When a coastal plain is young, the surface is smooth, the 
drainage is simple, and the shore line is regular. If the con¬ 
tinental shelf is gently sloping, offshore bars will soon form. 

The greatest coastal plain in the world forms the northern 


PLAINS 


487 


and western parts of Siberia. It has a maximum width of 
more than one thousand miles. 

On the shores of the Bay of Bengal is a narrow coastal 
plain not more than fifty miles wide. On the western coast 
of the United States, coastal plains are unimportant, al¬ 
though there are some narrow plains like that at Los Angeles. 
On this coast, continental shelves are scarcely noticeable. 

The Atlantic Coastal Plain. — This interesting plain lies 
along the Atlantic coast from New York to Florida and 
extends from Florida around the Gulf of Mexico as the Gulf 
Coastal Plain. Its width varies from one-half mile to five 
hundred miles. 

The rock waste forming this vast deposit, which in places 
is several hundred feet thick, came mostly from the eastern 
slope of the folded Appalachian Mountains, and this supply 
ceased only when the mountains were base-leveled. 

During the period of deposition eastern North America 
was submerged as far as the ancient shore line that is located 
quite near the “ fall line ” (Figure 227). 

The period of deposition on the coastal plain came to an 
end when the continent emerged as far as the outer limit 
of the present continental shelf. 

Many centuries of weathering and stream erosion fol¬ 
lowed the period of deposition, during which the streams 
flowing into the Atlantic Ocean cut deep valleys in the soft 
deposits of the then new coastal plain. These can still be 
traced to the outer edge of the present continental shelf. 

This period of erosion ended for the eastern half of the 
coastal plain when it was again submerged, moving the shore 
line to about its present position, drowning the valleys that 
had been eroded, and giving us New York, Delaware, and 
Chesapeake Bays as well as the bays and sounds south of 
them, and many smaller bays like those of the Navesink, 
Toms River, and Great Egg River of New Jersey. 

The shore of the coastal plain is low and marshy, but the 
land rises gently to the fall line. The strata beneath the 


488 


NEW PHYSIOGRAPHY 


soil are similar to those of the continental shelf and contain 
many marine fossils that prove the former submergence 
of the region. 

In the Carolinas rice is raised in the marshes. Between 
the marshes are wide areas of sand, of little value for agri¬ 
culture, which are chiefly occupied by pine forests. Farther 
inland the soil is fertile, and much cotton is raised; and in 
certain localities gardeners maintain successful truck farms. 



Fig. 270 . — The Great Pine Plains of Southern New Jersey 
A part of the Atlantic coastal plain, much of which is like this. 


At the western border of the Atlantic Coastal Plain the 
land rises somewhat abruptly to the Piedmont Plateau. 
The rivers of this region usually have falls or rapids where 
they descend from the plateau to the plain, which furnish 
water power and mark the head of navigation. 

Ancient Coastal Plains. — Geikie well says, “ Only where 
the sediment is strewn over the sea floor beyond the limit 
of breaker action is it permitted to accumulate undisturbed. 
In these quiet depths are now growing the shales, sand¬ 
stones, and limestones which by future terrestrial revolution 
will be raised into land, as those of the past have been.” 





PLAINS 


489 


A study of the subject will convince one that North 
America began as a F-shaped area about Hudson Bay (a 
part of the area of metamorphic rocks shown around the 
shore of Hudson Bay in Figure 136) and that the continent 
has grown to its present size through the emergence of 



Fig. 271 . —Ruins op the Temple op Jupiter Serapis 

Note the rough surface of the columns due to the lithodomi. 
Photo by Ewing Galloway. 


successive continental shelves, each one of which was for a 
time a coastal plain. 

Some of the former coastal plains still retain enough of 
their original characteristics to enable us to recognize them 
as “ ancient coastal plains.” 

Other portions of them are now great mountain ranges, 
and still other portions have been great mountain ranges 
but are now so worn down that we call them plains. 





490 


NEW PHYSIOGRAPHY 


Changes of Level. — There are many accounts in both 
modern and ancient history of such changes in the relative 
level of the land and sea as would submerge a coastal plain 
or convert a continental shelf into a coastal plain; and there 
are many geological evidences that such changes occurred. 

The ancient temple of Jupiter Serapis at Pozzuoli, near 
Naples, is known to have been on dry land in a.d. 235. 
When it was rediscovered in 1749, the bases of its remaining 
upright columns were buried in marine sediments to a depth 
of twelve feet above the floor of the temple. For a distance 
of nine feet above the sediment, the lithodomi, or stone- 
house animals, had bored holes in the columns and lived 
in them, causing the dark bands seen in the columns shown 
in Figure 271. 

The double caves shown in Figure 104 were cut by the 
waves. When the upper cave was cut out, it was in about 
the same position with respect to sea level that the lower 
one now occupies. 

These illustrations indicate recent changes of level. The 
finding of the skeleton of a whale in glacial gravels near 
Lake Champlain indicates an earlier change, and the-remains 
of sharks’ teeth and marine animals in the sedimentary rocks 
of our mountains indicate still earlier changes. Finally, it 
must be recognized that the very existence of dry land is 
evidence of change of relative level of land and sea, for 
without it erosion would long since have reduced the land 
to sea level. Each of these changes was necessarily accom¬ 
panied by a change in the location of the shore line. 

Some of the changes may have' been due to depressions of 
the ocean bottom, which would allow the water to settle 
away from the land; others to the accumulation of sedi¬ 
ment, or lava, on the sea bottom, which would cause the 
water to overflow the land; still others to the withdrawal 
of the sea water to form a continental glacier; and yet others 
to the uplift of the lands. 

Such changes have repeatedly exposed fresh areas of the 


PLAINS 


491 



ocean floor, forming marine plains, or have tilted land, drain¬ 
ing lakes and forming lacustrine plains. 

It is not always possible to determine whether it was the 
surface of the land or that of the sea that changed its dis¬ 
tance from the center of the earth; land may be submerged 
either by the sinking of the land or by the rising of the sea; 
and it may emerge because of the subsidence of the sea or 
the rising of the land. Any radial motion of either the land 


Fig. 272 . — The Lacustrine Plain 

In the valley of the Red River of the North. The points on the sky line are houses. 

surface or the sea surface, or of both of them at the same 
time, will cause land either to emerge or to be submerged, 
except when both surfaces move in the same direction at the 
same rate. 

Lacustrine Plains. — When a lake is destroyed by either of 
the methods described on pages 451-452, the lake bottom be¬ 
comes a lacustrine plain. The surface is usually very level 
as shown in Figures 272 and 273, and the soil is usually 
fertile. 

The black muck soil of these lake bottoms in Michigan, 





492 


NEW PHYSIOGRAPHY 


New York, and elsewhere is especially valuable for celery 
and onions. 

In places around the former shores of extinct lakes there 
are beach deposits of sand and gravel. These are of little 
value for agriculture but of great value for building purposes, 
especially if they are near a large city. Nearly all of the 
sand and gravel used in Syracuse, New York, comes from 
such deposits. 

Lacustrine plains are usually small, though some of the 
large ones have an area of several thousand square 
miles. 

It is stated that there are about eight thousand lakes in 
Minnesota and that about one-half of them will become 
farm land by natural processes in half a century or less. 
Michigan, Wisconsin, Connecticut, and other states once 
covered by the glacier also have many similar lakes, and 
there are many more on our coastal plains all of which will 
eventually become farm lands. 

Lakes in the last stages of existence present interesting 
problems to engineers with a wagon road or a railroad to 
build across them. A floating mat of vegetable matter 
mixed with wind-blown dust often spreads out from the 
shore (Figure 249) often covering the whole lake. The mat 
reaches a thickness of fifty feet in some cases, but is usually 
much thinner. 

In one instance the floating mass was strong enough to 
support the railroad embankment and the track; but when 
the first train passed over, the grade, track, and train slowly 
disappeared under water. In another instance a railroad 
was built across a lake in Minnesota that was entirely cov¬ 
ered with a floating mat four feet thick. It was used as a 
pasture by the owner. Investigation showed that there was 
twenty feet of water beneath the mat. 

When a railroad line was built across the Apalachicola 
River in Florida in 1907, a long stretch of wooded swamp 
and open marsh was crossed in which there were mud and 


PLAINS 493 

vegetation deposits so deep and so soft that spliced piles 170 
feet long did not reach a solid foundation. 

Playas. — In arid regions occasional cloudbursts cause 
great torrents to flow over the level land in sheets and to 
accumulate in large, shallow, and temporary lakes called 
playas . The largest of these is in the Black Rock Desert, 
Nevada, which has an area of about 500 square miles 
although rarely reaching a depth of one foot. When the 
water evaporates, the basins are covered with beds of fine 



Fig. 273 . — The Floor of Ancient Lake Bonneville in Utah 
The hills and mountains are nearly buried by accumulated sediment. 


sand and clay, sometimes mixed with crystals of gypsum 
and salt. This deposit shrinks and breaks into angular 
pieces separated by wide cracks, as shown in Figure 274, 
which are known as mud or sun cracks. Similar cracks are 
seen everywhere when the temporary pools dry up. 

Salinas. — Where lakes have been destroyed by evapora¬ 
tion — since only the water evaporates — all of the dissolved 
mineral matter that was in the water is deposited on the 
lake bed. Such plains are called salinas. As a rule these 
plains are not fertile, but often they contain valuable deposits 
of salt, soda, or borax. Sometimes they may be made fertile 
by irrigation. 







494 NEW PHYSIOGRAPHY 

Soda, salt, and borax have been found in the Great Basin 
and particularly in the Imperial Valley. (See Figure 247.) 

In Bolivia there is a salina several thousand square miles 
in area. It is a level white plain covered with a layer of 


Fig. 274 . — Mud Cbacks 

salt four feet thick. Lake Titicaca is approaching the salina 
condition. 

Floor of Lake Agassiz. — One of the levelest regions 
in the world, and one of the greatest lacustrine plains, is 
the valley of the Red River of the North, which flows north 
between the states of Minnesota and North Dakota and 
empties into Lake Winnipeg in Canada. (See Figures 213, 
268, 269, 272, and 279A.) 















PLAINS 


495 


It is the floor of former Lake Agassiz, which existed while 
the continental glacier blocked the drainage lines toward the 
north. The soil here is fine-grained and rich, so that it pro¬ 
duces enormous quantities of excellent wheat. 

Floor of Lake Bonneville. — Great Salt Lake is a shrunken 
remnant of a greater lake known as Lake Bonneville, which 
once occupied the eastern portion of the Great Basin. The 
sediments deposited in this lake, which was as large as Lake 
Huron, filled the valleys between north and south mountain 
ranges, forming many small lacustrine plains (Figure 273). 

Other Lake Plains. — In several places along the south 
shore of the Great Lakes bodies of water accumulated, 
toward the close of the glacial period, between the ice front 
and the high land to the south. The sediments deposited in 
these lakes filled the irregularities in the lake bottom; and 
when the water disappeared with the melting of the ice, a 
number of important lacustrine plains were exposed. The 
prairies of northern Illinois were once covered by the waters 
of Lake Chicago, an extension of Lake Michigan. They are 
now lake plains. In New York State, a southern extension 
of Lake Ontario gave us the lacustrine plain that extends 
from the Mohawk Valley to Syracuse. This plain provided 
a favorable location for the Erie Canal and the New York 
Central Railroad. It was also used by the early settlers of 
the west as the main highway toward their new homes. 

Some of the former lakes of the group of Finger Lakes of 
New York State are now lacustrine plains; one of them, 
the valley of Mud Creek, just west of the Canandaigua 
Lake, is sixteen miles long and from one-half to one mile 
wide. It is level and is one of the most fertile regions of 
the state. 

Flood Plains. — Graded streams cut away their banks and 
widen their valleys. 

The lower part of a stream reaches the graded condition 
first. It is in this lower part that the stream first develops 
a valley that is wider than it can fill, except at flood stage. 


496 


NEW PHYSIOGRAPHY 


At other times the stream occupies the channel ( i.e ., the 
deepest part of the river), leaving a flat, swampy area on 
either side of its meandering course (known as the flood 
plain ). 

During floods, when streams overflow their banks, they de¬ 
posit sediment on the flood plain, thus building them higher 
above the low-water level. As the water leaves the channel, 
its velocity is checked and most of the sediment is deposited 
near the channel. This results in a smooth, nearly level sur- 



Fig. 275. — Flood Plain of the Grand River at the Mouth of the Gunnison, 
Grand Junction, Colorado 

The Grand River flows through a barren region, much of which is reclaimed by water 
led from the Grand by irrigating canals. 

face that slopes away from the channel, ending in marshy 
“back swamps” at the outer edge of the flood plain. 

The materials forming the flood plains are not arranged 
in continuous horizontal strata, but are exceedingly irregu¬ 
lar, owing to the meandering of the channel and to the fact 
that during floods a river often erodes new channels to 
great depths. As the flood subsides, the depressions thus 
formed are filled with layers of sediments which sometimes 
differ from those eroded in fineness and also in inclination. 
In this way the nearly horizontal original deposits of flood 
plains are cut away first in one place and then in another. 












PLAINS 


497 


The flood plain of the Mississippi is about 80 miles wide 
at Greenville, Mississippi, gradually decreasing toward the 
north until at Helena, Arkansas, it is but 20 miles wide. 
The total area of the Mississippi flood plain is about 30,000 
square miles. 

Piedmont Alluvial Plains. — A second type of stream- 
made plain is formed when steep tributaries flowing from 
mountains bring more rock waste to the main valley than 
the master stream can carry down the gentler slope. 

Under these conditions tributaries build alluvial fans 
which, joining those of neighboring tributaries, may make a 
plain several hundred miles long, but very narrow. The 
master stream may be crowded to the farthest side of the 
valley by the fan, as has happened to the San Joaquin and 
Sacramento Rivers in California and the Po in northern 
Italy. 

Figure 276 shows the piedmont plain formed by the 
streams from the San Bernardino Mountains in California. 
Note the fan shape of the contour lines at the mouth of each 
tributary. 

In arid regions such plains are usually the sites of villages, 
because water may be obtained either from mountain streams 
or from wells. Most of the settlements in Utah are situated 
on a plain of this type, lying west of the Wasatch Mountains, 
a range about 200 miles long that borders the great basin on 
the east. There is little slope along a line parallel to the 
mountains, and roads and railroads usually follow the 
direction of the range. At right angles to the range, the slope 
may be quite steep. The contours on Figure 276 show the 
slope in that instance to be between 250 and 300 per mile. 

Erosion Planes. — The first effect of erosion is to make the 
surface of the earth more uneven, because weaker rocks are 
worn away faster than the more durable ones, so that lines 
or areas of weak rocks become depressions, and then stream 
valleys. This unevenness must increase as long as any 
weak rock remains above base level. 


498 NEW PHYSIOGRAPHY 



Fig. 276. — A Piedmont Alluvial Plain 
Scale about one mile per inch. Cucamonga Top. Sheet, U. S. G. S. 
































































































































PLAINS 


499 


When a mass of weak rock reaches • base level, its erosion 
ceases. After the weak rock is gone, there is only resistant 
rock to be worn down. The erosion is therefore slower; 
but if time enough is allowed this resistant rock will also be 
worn down to base level, and the final term of the erosion 
cycle will be a featureless mathematical plane. 

If, as has usually happened, the erosion of the durable 
rock ceases before the mathematical plane is quite reached, 



Fig. 277. — Till Plain near Columbus, Ohio 

The mantle rock here is unstratified glacial drift (till) spread smoothly over somewhat 
uneven bedrock. Many prairies are of this origin. (Photo by Professor E. Orton.) 


we call the region an erosion plane or a peneplain, or perhaps 
better, a peneplane. The word peneplane means “almost 
a plane.” 

Since several agents may produce level districts by erosion, 
we may have surfaces that differ slightly and that may be 
distinguished as marine, river, glacial, or eolian erosion 
planes. 

Erosion planes present a very even sky line broken only 
by occasional masses of resistant rocks or of rocks that 
were in a favorable location with respect to streams. These 






500 


NEW PHYSIOGRAPHY 


masses sometimes rise high above the general level, and are 
then called monadnocks. Figure 300 is a cross-section of the 
uplifted peneplain of New England through Mount Monad- 
nock, the type of such relict mountains. There are several 
monadnocks in New England besides the one which bears 
the name, and there are others in Georgia and elsewhere. 

The mantle rock of a peneplain may consist of sediments 
deposited in former lakes and rivers, or of drift deposited 
by the continental glacier; but these local deposits do not 
indicate the true history of the region. It is the inclination 
of the strata of bedrock that shows the size of the folds 
removed and the extent of the work of erosion, and it is the 
erosion of the bedrock, rather than differences in the thick¬ 
ness of the mantle rock, which gives the region its plainlike 
characteristics. 

Glacial Plains. — In some regions a continental glacier 
spreads till or bowlder clay over large areas of somewhat 
irregular bedrock, producing level lands like the well-known 
till plain of northwestern Ohio (Figure 277). 

Water from melting ice carries rock waste from the front 
of the glacier and deposits it in imperfectly assorted layers, 
forming an outwash plain in front of a continental glacier, 
and valley train in front of a valley glacier. 

The Great Plains. — These extend from the Gulf of 
Mexico to the Arctic; from Minnesota and Iowa on the east 
to the Rocky Mountains. During the subsidence of the land 
that occurred before the Rocky M ountains were formed, the 
sea spread over the continental interior from the Gulf of 
Mexico to the Arctic Ocean, a distance of about three thou¬ 
sand miles, covering an area that was one thousand miles 
wide in places. During the retreat of the sea, muds spread 
over the bottom and sands spread over the muds, beach 
sand being the last deposit. These deposits reached a 
thickness of several thousand feet. (See Figure 279A.) 

While this condition prevailed, a great mountain move¬ 
ment began which eventually formed the Rocky Mountains 


PLAINS 


501 


and the Andes. During this movement, the strata that 
formed the Great Plains were tilted and bent until at the 
foothills of the Rockies the Great Plains are about six thou¬ 
sand feet above sea level and slope gently down nearly to sea 
level at the Mississippi River, some eight hundred miles 
away. The average slope, being less than two per cent, is 



Fig. 278. — A Small Butte 
Jail Rock, Cheyenne County, Nebraska. 


hardly noticeable. The deposits thus tilted determine the 
slope of the Great Plains in Alberta, Saskatchewan, and the 
Dakotas. 

As the mountains grew higher and steeper, the eroding 
power of the streams on their slopes increased, and great 
quantities of rock waste were brought down to the plains. 
Here the material was dropped, filling the valleys. 

The Missouri, the Platte, the Arkansas, and many other 
streams are still overloaded (page 431) and have failed to 






502 


NEW PHYSIOGRAPHY 


carry their burden half-way across the plains. Some of these 
streams, though a mile or so in width, are too shallow even 
for canoeing. 

Since an overloaded stream often shifts its course, there 
are numerous deposits found in ancient river channels and 
also on the floors of ancient lakes , some of which covered large 



Fig. 279. — Red Butte, Wyoming 
The upper layer is gypsum, thirty feet thick. 


areas of the plains. Other deposits on the plains were made 
by winds. (See pages 278 and 282.) 

These deposits reached a thickness of 500 feet in places. 

Streams have cut deep valleys through them which show 
that they consist of unconsolidated gravels, sands, and clays. 
These substances are easily eroded by the cloudbursts to 
which the region is subject. 

Erosion of the High Plains. — In parts of Dakota and 
Montana along the edges of the high plains formed by these 
deposits, the land is being eroded so rapidly as to make 
travel over the regions extremely difficult, if not impossible. 






PLAINS 503 

These regions are known as the “Bad Lands/’ the Mauvaises 
Terres of the early explorers. (See Figure 157.) 

The rolling character of the plains is due to the erosion that 
has taken place in the long interval since the deposits emerged 
from the sea. 

It is estimated by various authorities that between five 
and seven million years have elapsed since the Great Plains 
became dry land. That much material has been eroded is 
proved by such outliers or buttes as Jail Rock in western 
Nebraska (Figure 278) which shows that the whole of the top 
layer of sandstone has been worn away with about an equal 
thickness of the shale beneath it. 

Figure 279 shows the same phenomenon in another locality. 

In each of the illustrations erosion of the interstream 
spaces is shown. Near the mountains the stream valleys are 
from 50 to 100 feet deep. 

QUESTIONS 

1. What spherical bodies owe their shape to gravitation? 

2. Do solids assume a spheroidal form when rotated? 

3. What benefit does man derive from the relief of the earth? 

4. Why does the finding of the skeleton of a whale in glacial 
gravels near Lake Champlain indicate'a change of level of the land 
in that region? 

5. What sort of evidence would show that a given plain was of 
lacustrine rather than of marine origin? 

6. Are all lacustrine plains fertile? Why? 

7. What facts prove that the Atlantic Coastal Plain was once 
a part of the continental shelf? 

8. What evidence have we that the continental shelf was once a 
part of the continent? 


CHAPTER XXIII 


PLATEAUS AND MOUNTAINS 
Plateaus 

Both plains and plateaus have broad, relatively smooth 
upper surface and usually horizontal bedrock. 

A plateau is a highland — high in contrast with some 
near-by lowland; but when we compare the elevations of 
plains and plateaus that are widely separated, we learn that 
some plains are much higher than certain plateaus; for 
example, the Piedmont Plateau, between the Appalachian 
Mountains and the Atlantic Coastal Plain, is much lower 
than the plains of the Mississippi Valley; and the Appala¬ 
chian Plateau has an altitude of 2500 to 5000 feet, whereas 
the Great Plains east of the Rocky Mountains reach an 
altitude of 6000 feet. Although plateaus are, as a rule, 
higher than plains, it is not possible to distinguish between 
them on the basis of altitude. Because of the steep descent of 
about 200 feet along the eastern edge of the Piedmont 
Plateau, people on the plain at the foot of the descent 
naturally speak of the elevated surface as a highland or a 
plateau (See Figure 279A.) 

The only possible distinction seems to be based upon the 
relative altitude of the plateau and the surrounding regions. 

A plateau is a region of broad summit area that is conspicu¬ 
ously higher than adjoining land or water on at least one side. 

Changes of Level. — The changes in the relative level of 
portions of the earth’s crust that involved the greatest 
movement were those that formed the continents and ocean 
basins. In their formation, millions of square miles of the 
earth’s crust were moved thousands of feet vertically. 

504 


Fig. 279 / 1 . —Physiographic Region of the United States. 


PLATEAUS AND MOUNTAINS 


505 













506 NEW PHYSIOGRAPHY 

When plains emerge from the sea, extensive areas are lifted 
a few feet. In mountain making, smaller masses are moved 
thousands of feet. In plateau making, thousands of square 
miles are moved thousands of feet, as was the case with the 


Fig. 280. — In the Middle Distance, the Kaibab Plateau, Nine Miles Away 

western plateau of North America and also with the great 
plateau of Tibet which is higher than most mountains. 

In magnitude of crustal movement, plateau making stands 
next to the movement that formed the ocean basins and continents. 

Formation of Plateaus. — (1) Plateaus may be formed by 
successive lava flows, as were the Columbia Plateau of 
Oregon and Idaho (Figure 279 A), and the Deccan Plateau of 
India. (2) They may be formed by vertical forces that either 
elevate the plateau or depress the surrounding land, as were 



PLATEAUS AND MOUNTAINS 507 

the fault plateaus of Arizona. (3) They may be simply 
remnants left when adjacent land was eroded. The Kaibab 
Plateau (Figure 280) is such a remnant. 

Faults. A. break in the rock along which the rock of one 
side has been elevated or depressed is called a fault. 

Figure 281 is a sketch of a fault in a coal seam in Indiana. The 
rock was broken along the surface 1, 2, 8, 4, called the fault 
plane, and the left-hand side was uplifted — or the right side de¬ 
pressed — so that the coal seam was no longer continuous. Miners 



Fig. 281 . — Block Diagram op a Fault in a Coal Seam, Indiana 
Modified after Ashley. 


working such a seam as, for instance, the right side of the fault 
would find that the coal ended and was replaced by shale when they 
reached the fault. Experience has taught them not to abandon the 
mine in such a case, because it is merely “a fault in the seam.” 
How could they find the seam of coal on the left of the fault? 

A newly formed fault often leaves a crack in the earth’s surface 
that marks its location and is called a fault trace (Figure 318). 

Fault Plateaus. — The plateaus of northern Arizona, cut 
by the Colorado River, consist of a series of broad level areas, 
each of which is separated from the next by a steep cliff, 
giving the region the appearance of a giant stairway. One 
of the cliffs. Hurricane Ledge, is 1800 feet high. Such pla- 





































508 


NEW PHYSIOGRAPHY 


teaus are formed by faulting of the bedrock. They probably 
reached their present altitudes through many slight changes 
of level continued for long periods of time, rather than by 



a single mighty effort. They are called fault plateaus. 
(Figure 282.) 

In Figure 282 the plateaus A, B, and D have evidently been 
formed by a stretching force which faulted the rock and allowed 
the blocks B and C to drop down, exposing the fault cliffs EF , 
GH, etc., which separate the “steps.” 


Erosion of Plateaus. — The streams draining a young 
plateau flow down the steep side or sides of the plateau 



Fig. 283. — Diagram of a Young Plateau 


bordered by lowlands and have great eroding power, cutting 
canyons and deep valleys that are the most striking features 
of the plateau. The Colorado River has eroded the Grand 




PLATEAUS AND MOUNTAINS 509 

Canyon, which is 125 miles long and in places more than a 
mile deep. As time goes on, however, the river will ap¬ 
proach base level; weathering will then change the canyons 
into broader U-shaped valleys, and then into mature valleys, 
leaving only narrow ridges of the plateau surface similar to 


Fig. 284. — Appalachian Plateau of West Virginia 

those seen in Figure 284. The plateau is then said to be 
mature. 

The Appalachian Plateau , which extends along the western 
border of the Appalachian Mountains from the Hudson River 
to Georgia, is mature. The evidence of its once continuous 
upland surface lies in the fact that the tops of its numerous 
ridges form a straight and nearly level sky line as shown in 
Figure 284 of the Appalachian Plateau in West Virginia 
and also by the fact that the layers of bedrock correspond 




510 


NEW PHYSIOGRAPHY 


exactly on the opposite sides of each valley as shown in the 
cross-section, Figure 285. 

The eastern margin of this plateau is higher than the Appa¬ 
lachian Mountains east of it. 

The region has abundant rainfall which, with the steep 
slopes, has given the streams great power of erosion. Their 



Fig. 285. — Cross-Section of the Appalachian Plateau 
near Charleston, West Virginia 

Scale, one inch equals two miles. 


valleys are separated by narrow ridges often 1000 feet high 
and too steep for agriculture. The bedrock exposed in their 
valleys has revealed valuable mineral deposits, iron ore, 
coal, oil, and natural gas. The region is well forested, as 
Figure 284 shows, and lumbering is an important industry. 



Base level 

Fig. 286. — Diagram of an Old Plateau 
Showing a Butte and part of a mesa. 


The only cities in the region owe their growth and develop¬ 
ment to the resources of the region. All of the towns, roads, 
and railroads are located in the valleys. 

Old Plateaus. — If a plateau should be completely re¬ 
duced to base level, it would become a plain, showing no 
evidence of the existence of the plateau. 

To accomplish this, however, it would be necessary that all 













511 


PLATEAUS AND MOUNTAINS 

parts of the plateau should be eroded at the same rate, which 
seldom happens. We have seen that erosion was always 
more rapid along the water courses than on the areas between 
the streams, and also that certain rocks were eroded much 
more rapidly than others. 


This unequal erosion results in outlying piles of rock like 
Antelope Butte (Figure 287). These piles tell us that the 



Fig. 287. — Antelope Butte 

These are monuments which show that much rock has been eroded. 


region is old and worn down, and they preserve a record of 
the kinds of rock that formerly covered the region. 

In New Mexico there are a number of elevated areas that 
have been preserved, either because of the durability of the 
upper layer of rock, or because of their location with respect 
to drainage lines, and which show that many hundreds of 
feet of rock have been removed and that the region was 
formerly a plateau. These flat-topped areas are called mesas , 
if large with nearly vertical sides, and buttes if small (Figures 
278, 279, and 287). 



512 


NEW PHYSIOGRAPHY 


Economic Importance of Plateaus. — High plateaus are 
colder and usually more arid than the adjoining lowland. In 
tropical regions this is an advantage, and the upland is 
usually an important agricultural region. For example, the 
plateau of Mexico furnishes the northern grains to a region 
where semitropical products abound on the lowland. In 
temperate climates the low temperature is a disadvantage. 
In the plateau of Tibet the great elevation causes the 
climate to be almost arctic, and much of the region is 
abandoned to wild animals and tribes of nomads. The 
centers of the settled and agricultural population of Tibet 
lie in the south. Some of the deeper valleys here are fertile 
and warm enough to produce two crops a year. 

Arid Plateaus. — The depth of the river valleys, even in 
moist plateaus, tends to lower the level of the ground water, 
thus increasing the difficulty of getting water. In regions 
of limited rainfall, therefore, plateaus are less suited to agri¬ 
culture than plains. 

Some farms flourish on our arid southwestern plateaus 
near the mountains, where streams may be used for irriga¬ 
tion, and some other sections are fair grazing lands; but as 
a whole the region is almost unoccupied, just as Tibet is. 

Mountains 

Mountains cannot be distinguished from hills on the 
basis of absolute altitude. The Alleghany Mountains are 
much lower than the Black Hills, but they are the most 
conspicuous elevations in their section; whereas the Black 
Hills are “ dwarfed by the Bocky Mountains.” 

Some mountains, like Pike’s Peak, consist of a single sharp 
summit or peak; others consist of long ridges. A ridge is 
an elevation having much greater length than breadth. 

A mountain range is a ridge or a group of parallel ridges , 
formed at the same time. 

A mountain chain is a group of approximately parallel 
ranges formed at different times. 


PLATEAUS AND MOUNTAINS 


513 


The term cordillera is applied to groups of mountain 
chains and ranges; for example, the cordillera of the western 
United States includes the Rocky Mountain chain, the 
Sierra Nevadas, and the Coast Range. 

In appearance mountains and plateaus are very much 
alike when viewed from the lowland, and this leads, occasion¬ 
ally, to plateaus being called mountains, particularly when 
the plateau has been dissected to some extent as has that 
part of the Appalachian Plateau commonly known as the 
Catskill Mountains. 

When one reaches the upland surface of a plateau, its 
resemblance to the mountain disappears; and its broad, flat 
surface, often with an area of many square miles, looks much 
more like the plain. 

The plateau has a large summit area; that of the mountain 
is small, forming a peak (Figures 298 and 303) or a ridge 
(Figure 297). 

The term mountain is used loosely by certain authors; 
some of whom define mountains as those parts of the earth’s 
crust in which the bedrock has been folded, compressed, and 
distorted by lateral pressure. 

Some of the great ranges of the world, like the Alps and 
the first Sierra Nevadas, were formed in this way, and the 
definition would be a good one if all mountains had this 
structure and if there were not both plains and plateaus 
that had the same structure. The following definition is 
more generally accepted: 

A mountain is an elevation which rises conspicuously above 
the surrounding country and which has a comparatively small 
summit area. 

Distribution of Mountains. — Several interesting facts 
concerning the distribution of mountains are well established. 
(1) Growing mountains are, as a rule, near the borders of 
continents rather than in the interior. (2) Most of the higher 
mountain systems are near the borders of the largest and 
deepest ocean, the Pacific. 


514 


NEW PHYSIOGRAPHY 


This arrangement gives the continents a short, steep slope 
bordering the Pacific and the Indian Oceans and a longer, 
gentler one toward the Atlantic and the Arctic Oceans. 

Origin of Mountains. — The more important processes 
by which mountains have been formed, with some of their 
characteristics, are presented in the following table: 


Pkocess 


1. Deposition. 

2. Intrusion. 

3. Unequal Erosion. . . . 

4. Faulting. 

5. Folding. 

6. Combined Action. . . 


Results 


Volcanic cones 
Domed mountains 
Monadnocks, 

dissected plateaus 
Block mountains 
Folded mountains 
Complex mountains 


Arrangement 


{ Single peaks or groups 
of peaks 

| Mountain ranges 


Deposition. — Volcanic peaks are the most mportant 
mountains formed by deposition. They are merely heaps of 
lava and ashes surrounding the opening in the earth from 
which the latter erupted. Mount Rainier (Figure 316) is 
one of the best illustrations in this country. It has been 
inactive for many years. Lassen Peak, in California, is 
the only active volcanic mountain in the United States 
(Figure 315). Mt. Vesuvius (Figure 312) is perhaps the 
best known, but it is very much smaller than the Japanese 
volcano Fujiyama (Figure 303), which has an altitude of 
12,365. feet, and than Aconcagua, a volcano in the Andes 
Mountains, between Chili and Argentina, which rises 
22,860. Some of the Hawaiian volcanoes have much greater 
peaks than Aconcagua; some of them rise from the bottom 
of the ocean as giant cones 30,000 feet high, with a circum¬ 
ference of 100 miles at sea level. These are some of the 
highest and most symmetrical mountain peaks in the world. 

In some sections deposits made by glaciers are 1000 or 
more feet high and are frequently called mountains. Sand 
dunes are also called mountains locally, some of them being 
600 feet high. 














PLATEAUS AND MOUNTAINS 


515 


Domed Mountains. — Several types of mountains were 
formed by forces that seem to have moved large masses of 
bedrock vertically without folding or compressing the strata. 
The Henry Mountains of Utah were domed, or lifted high 
above the surrounding country by the pressure of a mass of 
lava that did not reach the surface. 


In the sketch, Figure 288, A is a passage through the rock 
through which the lava has risen, forming a dike. At the top of the 
dike the lava spread sidewise 
forming a mushroom-shaped mass 
and pushing the bedrock up form¬ 
ing a great dome resembling those 
seen on many public buildings. 

The bedrock above the mush¬ 
room, XYZ, was stretched and 
broken as it rose. In the sketch, 
which represents only a slight 
elevation, the line XYZ is one- 
twentieth longer than XY, so 
that every twenty feet of the 
rock as it rested on the line XZ would have to stretch to twenty- 
one feet or else break. 



Fig. 288. — Diagram op a Domed 
Mountain 


The Black Hills of South Dakota were once great domes, as 
were the Elk Mountains of Colorado and other ranges of the 
Rocky Mountains; but in some of them the mass of lava is so 
large that we do not believe that it is an intrusion. 

Mountains Due to Unequal Erosion. — Certain moun¬ 
tains, like the mesas and buttes that stand as monuments 
marking the disappearance of former plateaus, owe their 
elevation not to folding or uplifting but to the more rapid 
erosion of surrounding, less resistant rocks. Mount Monad- 
nock was the first mountain of the kind to be described, 
and all mountains formed in this way are called monadnocks. 
Lookout Mountain is a remnant of an old plateau, and 
the “ temples ” of the Grand Canyon of the Colorado 
River, some of which are as high as Mount Washington, 

































516 


NEW PHYSIOGRAPHY 


were due to unequal erosion. The Cat skill Mountains 
of New York, which are really a part of the Appalachian 
Plateau (see page 510), and the present Appalachian Moun¬ 
tains were both formed by unequal erosion. 

Faulted or Block Mountains. — In the Great Basin region 
are ranges of such simple structure as to clearly show their 
origin. Figure 289 shows the structure and the surface of 
three of the basin ranges, A, B, and D. The bedrock was 
evidently fissured and faulted as was that of the neighboring 
fault plateaus (Figure 282); but here the blocks were tilted, 
either by uplifting the high edge of each block or by the sink¬ 
ing of the low edge, or perhaps by both motions occurring 



together. The fact that the wedge-shaped block C has 
dropped down suggests that a stretching force has been at 
work here also. 

Block mountains have a short, steep slope on one side and 
a long, gentle slope on'the other side. As Figure 289 shows, 
the layers of bedrock are usually parallel to the gentle slope. 

Oregon Ranges. — In southern Oregon are many block 
mountains that were formed so recently that erosion has 
not yet notched their summit lines, and the frequent earth¬ 
quakes of the region indicate that the ranges are still growing. 

Some of these ranges are 40 miles long, and in places they 
rise 2000 feet above the valleys. 

Block Mountains in Nevada and Utah. — These are older 
mountains, and are much eroded. The crests are notched 
and uneven, and the slopes are scarred with deep ravines as. 








Fig. 290. — Sketch of the Block Mountains of Nevada and Utah 


PLATEAUS AND MOUNTAINS 


517 







518 


NEW PHYSIOGRAPHY 


shown in Figure 290. Some of the ranges are 80 miles long 
and 20 miles wide, and their crests rise from 2000 to 7000 
feet above the valleys. 

Folded Mountains. — The great mountain chains of the 
world are records of former crustal motion, or displacements 
of the rocks, resulting from enormous compressing or uplift¬ 
ing forces. 

The origin of these forces is an unsettled question, but the 
following facts concerning them are generally accepted: 

1. They were formed of thick sediments that accumulated in great 
downward folds, or troughs, that from time to time have been formed in 
the sea bottom near the continents. 

2. These sediments were crushed together and folded so as to form 



Fig. 291. —• Cross-Section of the Jura Mountains 


mountain ranges, by lateral or sidewise 'pressure at right angles to the 
shore line, that shortened the layers. 

The Jura Mountains. — One of the best examples of simple 
folded mountains is the Jura Mountains between France 
and Switzerland. They consist of a series of ridges formed of 
sedimentary rock that contains marine fossils. The rocks 
were originally horizontal, but are now bent and folded so 
that the layers are parallel to the mountain slopes as shown 
in Figure 291. Most folded mountains also show faulting 
of the strata. Each ridge consists of layers of rock that form 
an arch. Such an upward fold, or arch, is called an anticline 
(Figure 292). 

Each valley consists of a downward fold or inverted arch 
of the same layers that is called a syncline (Figures 291, 293, 
and 295). 

The Jura Mountains have been only slightly eroded. The 
upper layers on top of the ridges have been removed and the 



PLATEAUS AND MOUNTAINS 519 

valley floors covered with rock waste, but the mountains 
are still young. 

The Appalachian Mountains. — The folds of these moun¬ 
tains are not at all like the simple curves of the Juras; but 
in the southern part where the greatest effect was produced 
by the lateral pressure, the folds have been crowded together 
so that two or more simple folds have formed compound 
folds and in some cases have been overturned toward the 



Fig. 292. — An Anticline near Hancock, Maryland 


west. In addition to this, the quartzites and other hard 
rocks have been faulted and thrust over the rocks west of 
them, crumpling the shales and weaker rocks as they moved. 
This is best shown at the left in Figure 294. In some of these 
faults the displacement is several thousand feet. 

Figure 295 is a cross-section of the Appalachian folds in 
Pennsylvania where the anticlines and synclines formed open 
folds like those of the Juras; the section is based upon the 
data given in the Harrisburg Pennsylvania Folio, United 
States Geological Survey. 

Examination of these ridges shows that the Pocono sand- 





520 


NEW PHYSIOGRAPHY 


stone forms the summit and the east side of Peters Mountain 
and also the summit and west side of Second Mountain as is 
shown in Figure 295, indicating that the Pocono sandstone 



Fig. 293. — A Syncline in Shale, Upton, Pennsylvania 


dips down beneath Third Mountain, forming a synclinal 
fold, as shown in the same figure. 

Undisturbed sedimentary rocks lie in horizontal layers. 
These were thus folded at the close of the paleozoic era. 



Fig. 294. — Cross-Section op the Folds op the Southern Appaeachains 
By the U. S. G. S. 


The lines of dashes marked MM (Figure 295) suggest the 
size of the anticlines that formed these ancestral Appalachian 
Mountains. The mountains have disappeared, but the record 
of the synclines proves that they once existed. 











PLATEAUS AND MOUNTAINS 


521 


If we were standing on the top of Blue Mountain (Figure 
295), looking west over Second, Third, and Peters Moun¬ 
tains, we should see that these mountain tops are at about 
the same level and that filling the valleys between the moun¬ 
tains would form a plain that would slope gently down 
toward the east. 

The Potsdam sandstone that forms the summit and west 
side of Blue Mountain is one of the most durable of the 
sedimentary rocks, but it has been worn down to a lower alti- 





Fig. 295. — Cross-Section op the Appalachian Mountains near 
Ellendale, Pennsylvania 


tude than the less durable rocks of the other summits, and 
all four of the summits have been eroded so as to conform 
closely the slope of the line P-Pl. The conclusion from the 
facts is that the ancestral Appalachian Mountains were 
base-levelled at P-Pl. 

By elevation, the streams of the region were rejuvenated, 
and stream erosion has since been wearing away the shales 
between the lines of resistant rocks, forming the valleys of 
the five creeks of the section and leaving the resistant rocks 
as mountain ridges. 

The present Appalachians were formed by erosion made 
possible by a vertical force, whereas the ancestral Appala- 










522 


NEW PHYSIOGRAPHY 


chians were formed by a lateral force that acted westward 
from the Atlantic. The top of Third Mountain was once 
the bottom of a great valley or syncline, and so is called a 
synclinal mountain. 

The Rocky Mountains. — Like all great mountain chains, 
the Rocky Mountains were formed from thick sediments 
that accumulated in a great synclinal fold in the bottom of 
the sea then covering interior North America. These sedi¬ 
ments were folded by lateral pressure; but the folds are sim¬ 
pler than those of the Appalachians, and the slopes are more 
gentle. Compare the slopes of the layers in Figure 296 with 
those in Figure 294. 

The compressing force was much less important in the 



Fig. 296. — Cross-Section of the Rocky Mountains 
By the U. S. G. S. 


growth of the Rockies than a vertical force that raised them 
without crushing and folding the strata and that also ele¬ 
vated a large part of the area west of the Rockies, including 
the mountains of the Great Basin and the plateaus of Arizona 
and Utah. 

Figure 297 is a photograph of one of the ranges of the 
Rocky Mountain chain and shows several features that are 
common to most of the other ranges of the chain. Notice the 
irregular skyline with its many peaks. Mount Hesperus, the 
highest peak in the photograph, has an altitude of 13,200 
feet. The trees grow smaller as we climb toward the top, 
and finally disappear. The irregular line which marks the 
upper limit of the trees is known as the timber line. Above 
the timber line the bare rock ledges which give the mountains 
their name can be seen from great distances. The steep slopes 












Fig. 297. — La Plata Mountains of Southwestern Colorado 
Showing the characteristic ruggedness of the Rockies. Mount Hesperus has an altitude of 13,200 feet. (Photo by U. S. G. S.) 


PLATEAUS AND MOUNTAINS 


523 





1 1 ^ 

















524 


NEW PHYSIOGRAPHY 


of the sides of the mountains are cut by torrents that carry 
down rock waste, depositing it where the slope is gentler. 

Structure. — As shown in Figure 296 the central mass of 
the Rocky Mountains is granite or other similar igneous rock. 
On either side of the granite are layers of sedimentary rocks 
which have been pushed by the granite as on the right of 
the central mass, and compressed into gentle folds as seen 
at the left of the figure. 

Age. — The sedimentary rocks shown in Figure 296 were 
formed at the close of mesozoic time whereas those of the 
Appalachians were formed at the close of paleozoic. An 
entire geological era elapsed between the formation of the 
Appalachians and the Rockies. 

Cycle of Erosion in Mountains. — The Sierra Nevada 
Mountains, the Coast Range, and other ranges that border 
the Pacific and the Indian Oceans are still growing. 

Observation has taught us the following facts about the 
constructive forces that build mountains: 

1. The growth is usually slow and is accomplished through many 
slight uplifts, usually accompanied by earthquakes. 

2. In most instances the uplift is less than 20 feet, although oc¬ 
casionally a much greater uplift occurs. 

3. The movements are usually intermittent with variable in¬ 
tervals of time between them; for instance, violent earthquakes 
occurred in the vicinity of Valparaiso, Chili, in 1822,1835, and 1906. 

4. The present Sierra Nevada and other block mountains are 
growing by almost continuous uplifts, each so slight as to be hardly 
perceptible. 

These mountain-building forces originate in the interior 
of the earth; they are opposed by the agents of erosion which 
tend to tear down and destroy the mountains. The following 
facts about the destructive forms are obvious: 

1. Erosion begins as soon as the region emerges from the sea. 

2. Its action is feeble at first but increases in vigor and never 
ceases until the region is again submerged. 

3. Its vigor varies with changes in climate and steepness of slope. 


PLATEAUS AND MOUNTAINS 


525 


From the above facts it follows that: 

4. No mountain ever reaches the full height that the uplifting 
forces would have given it if not opposed by erosion. 

Three and one-half miles of rock has been eroded from the top 
of the Uinta Mountains, but it is possible that they were never 
much higher than they are at present. 

5. The altitude of any range at any time shows the extent to 
which the constructive forces have been more effective than erosion. 

6. If the uplifting forces cease for a sufficient time, the moun¬ 
tains will be destroyed. Any subsequent uplift will begin a new 
cycle of erosion. 

7. The time during which uplift prevails over erosion is called 
the period of growth; that during which erosion prevails is the 
period of decline. The cycle of erosion is the interval between the 
emergence of a land form and its erosion to a base level, but cycles 
are rarely completed; they are usually interrupted and a new cycle 
introduced. 

The Life History of Mountains. — The life history of 
a land form is a description of the stages through which 
it passes during a cycle of erosion. During the warfare of 
constructive and destructive forces, the characteristics of 
mountains change so that one may readily distinguish young 
mountains from old ones. 

The Rocky Mountains are younger than the Appalachians 
and differ markedly from them. For example, the Rockies 
have bare ledges and cliffs, with small talus slopes; the 
Appalachian rocks are usually waste-covered. The Rockies 
have a very irregular sky line; the Appalachians present an 
even sky line. The Rockies rise 8000 or 9000 feet above the 
platform on which they rest; the Appalachians 900 to 1200 
feet. Avalanches and landslides occur occasionally in the 
Rockies but not in the Appalachians. The streams in the 
Rockies are young; those in the Appalachians are more 
mature. 

Young Mountains (Figures 297 and 298) are characterized 
by irregular sky line, bare ledges, steep slopes, streams in the 
torrential stage, and the summit lines still in their original 


526 


NEW PHYSIOGRAPHY 


positions. During youth, avalanches, landslides, and earth¬ 
quakes occur at times. As a rule, lofty and rugged mountains 
are young. 

Which of the following characteristics do you recognize in Fig¬ 
ure 298? In Figure 297? 

(1) Irregular sky line. (2) Bare rock ledges. (3) Steep slopes. 
(4) The timber line. 

Would you describe the current in any streams on these moun¬ 
tains as sluggish? As swift? As torrential? 



Fig. 298. — The Jungfrau, a Young Mountain 

Note the U-shaped glacial valley in the background showing former extension of 
Alpine glaciers. 


How should their power of erosion compare with that of streams 
on plains? 

Would you therefore expect to find shallow or deep ravines on 
young mountains? 

Subdued Mountains, illustrated in Figure 299, have uniform 
slopes, low, rounded form, and few bare ledges; earthquakes 
or avalanches are rare or unknown; the sky line is more 




PLATEAUS AND MOUNTAINS 527 

regular than that of young mountains, but less regular than 
that of old mountains; water gaps and passes have de¬ 
veloped. 

Compare the sky line in Figure 299 with that of the young moun¬ 
tains illustrated. 

Are the slopes of the mountains in Figure 299 as irregular as those 
of young mountains? Are they as steep? 


Fig. 299. — Subdued Mountains in North Carolina 

Old Mountains. — As mountains grow old, the region 
approaches nearer and nearer to the erosion plane. Monad- 
nocks stand out here and there telling us of the vast work 
that has been performed by erosion, and occasionally we 
find the bedrock bent downward as is in Figure 295, proving 
that the region was once mountainous. Figure 300 is a 
cross-section of the New England erosion plane showing 
Mount Monadnock, a folded structure, which shows that the 
region was once mountainous and that it is now a region 









528 


NEW PHYSIOGRAPHY 


of old mountains. The region south of Lake Superior is 
also an old mountain region. 

The uplift and decline of mountains takes place so slowly 
that the change produced during a lifetime passes unnoticed, 
and men have come to think and speak of them as everlasting. 
History fails to give us assistance in determining the rates at 



Fig. 300. — Cross-Section of the New England Erosion Plane 


which these changes progress. Polybius’ description of the 
Alps as they were when Hannibal crossed them in 218 b.c. 
is practically a description of the Alps to-day. The Alps 
are still young mountains, and the lapse of 2000 years has 
not materially changed them. It is evident from these con¬ 
siderations that the life history of mountains cannot be ex- 



Fig. 301. — Mount Monadnock and the Erosion Plane 


pressed in terms of years or even in thousands of years, and 
that the time required to wear down our old mountains 
to the present erosion planes was very long. 

Climate of Mountains. — The snow-capped mountains of 
the torrid zone exemplify upon their slopes all the climatic 
changes that one would experience in traveling from the 
torrid zone to the polar regions. As one ascends, the palms 
and bananas of the torrid zone gradually disappear and are 
replaced by the deciduous trees and wild flowers of the tern- 














PLATEAUS AND MOUNTAINS 


529 


perate zone. These in turn are replaced by the cone-bearing 
trees, which, as the ascent is continued, become low and 
dwarfed; finally all trees disappear. Above this point 
grasses and bright Alpine flowers flourish; but these also 
disappear as the ascent continues, and the snow-clad top is a 
frigid zone in miniature. In a similar manner the forms of 
animal life that inhabit the bases of such mountains gradu¬ 
ally disappear and are replaced by forms which characterize 
the higher latitudes. 

The great variety in mountain climate is due to the fact 
that the vertical temperature gradient in air at rest is more 
than 1000 times as great as the average horizontal tempera¬ 
ture gradient. That is to say, the average annual tempera¬ 
ture decreases more than 1000 times as fast as one ascends 
as when one travels poleward. 

The timber line and snow line are more or less irregular, 
being usually higher on the sunny side of east and west 
ranges than on the shady side. In the equatorial region 
the snow line is about 18,000 feet above sea level, but its 
altitude diminishes as the distance from the equator in¬ 
creases, reaching sea level in the Arctic and Antarctic 
regions. 

Many ranges are subject to excessive rainfall or snowfall 
on the windward side; and where they cross prevailing winds, 
the climates of the opposite slopes are in sharp contrast. 
For example, on the western slope of the Sierra Nevadas, the 
moist wind is chilled as it rises, producing abundant rainfall, 
which supports forests; whereas the same wind on the eastern 
slope, having lost most of its moisture and being heated by 
compression as it descends, becomes a drying wind which 
takes moisture from the land, making it arid. 

Influence on Man and History. — Because of the difficulty 
of crossing mountain ranges, the difference in climate on 
the different sides, and the military advantages which they 
afford, mountain ranges are the natural boundary lines for 
nations. The Himalayas, which separate different races; the 


530 


NEW PHYSIOGRAPHY 


low Pyrenees, crossed by but a few roads and railroads; 
the Caucasus, the Alps, and the Andes all illustrate the 
tendency of nations to select mountain ranges for their 
frontiers. 

As the Indian and the pioneer gained a measure of security 
within their stockades, so a nation surrounded by mountain 
ramparts is in a measure secure from outside interference. 
It requires a greater incentive to cause outside nations to 
attack them than is required to lead them to attack nations 
not so surrounded. The elevation of their outposts enables 
them to see an approaching enemy that would be invisible 
on a plain, thus diminishing the chance of surprise. Narrow 
passes well fortified can be successfully defended against 
vastly superior numbers, because the invading army cannot 
approach the pass in line of battle and is met in small parties. 
The famous defence of Thermopylae illustrates this advan¬ 
tage. 

The soldier on the mountain meets a tired foe, and in hand- 
to-hand conflict this is an important aid. Artificial ava¬ 
lanches of bowlders have frequently decimated armies 
attempting to cross mountain passes. When Hannibal 
crossed the Alps, his losses through this kind of warfare 
contributed in no small measure to his ultimate defeat. 

Because of the security afforded, conquered races usually 
make their last stand in mountains and have frequently 
been able to maintain their position through long periods. 
Some of these people have maintained their individuality 
even to the present day, as the Basques, the Welsh, and the 
Highlanders. 

With the military advantage comes a degree of isolation 
which favors the development of a distinct type of civilization 
and an individual language, or dialect, in the region thus set 
apart from the rest of the world. This tendency is illustrated 
in the many small principalities which developed in Europe 
during the Middle Ages, several of which exist to-day; and 
in the fact that in the California valleys there were almost 


PLATEAUS AND MOUNTAINS 


531 


as many tribes of Indians having characteristic languages 
and customs as there were valleys between the mountains. 

The same isolation limits commerce and knowledge of the 
outside world, and compels the residents of mountainous 
regions to depend upon themselves for their wares and for 
their progress. If their number is small, as it is apt to be on 
mountain slopes where the struggle for existence is so 
strenuous, there is rarely progress in the ways of civiliza¬ 
tion, but instead there is often a retrograde movement. 
Mountaineers are proverbially conservative, using the same 
processes and following the same customs that their forebears 
used and followed. In the southern Appalachians we find 
excellent illustrations of this effect; here are peoples follow¬ 
ing habits and customs of the eighteenth century. Mining 
cities in mountains are exceptions. To them the sudden 
wealth brings all that is good and all that is bad in our mod¬ 
ern civilization. 

Mountains as Barriers. — Several conditions make it 
difficult even for man to cross our high mountains. (1) The 
steep slopes at low elevations are hard to climb; those at high 
elevations are worse because of the thin air. (2) More serious 
conditions are the low temperature, the penetrating wind, 
and the driving snowstorms, followed by the blinding glare 
of the sun. (3) Avalanches and landslides sometimes bury 
whole caravans in the Himalayas. In our own Cascades, 
an avalanche carried away an entire train of pullman cars. 
(4) Doubtless the most dangerous route over mountains in 
the world is the caravan route from India into western China. 
For several days’ journey the route is above the timber 
line where no grass can grow and where it is so cold that 
there are no inhabitants. Thousands of horses have been 
lost on this section through cold and hunger. 

Even the minor mountain ranges retard the exploration and 
settlement of a region. 

Mountain ranges are watersheds and rivers rarely cross 
them. The explorer who wished to cross them was obliged 


532 


NEW PHYSIOGRAPHY 


to obtain horses or carry his supplies on his back. This 
required complete reorganization of a party that was 
equipped to travel by canoe, and often caused the explorer 
to turn back. 

If an explorer follows rivers, shelter and food for many 
weeks may be carried in a canoe by one man; if he journeys 
over plains, he must carry food for his horses and the men 
who care for them as well as for himself; if he is to cross 
mountains, pack animals must be substituted for wagons, 
with further increase in the size of the party. 

There is no better illustration of this retarding action than 
that found in the early history of this country. Before the 
year 1600, European explorers had visited the mouths of the 
St. Lawrence, the James, the Mississippi, and the Rio 
Grande, and had visited California. During the next cen¬ 
tury the English explored and settled the Atlantic Coastal 
Plain, but made few attempts to cross the low ridges of the 
Appalachians; the French, during the same period, explored 
the St. Lawrence and followed the Mississippi to the Gulf. 
They established settlements along the routes which grew 
into towns still bearing French names, such as Detroit, 
Sault Ste. Marie, Fond du Lac, Prairie du Chien, St. Louis, 
and Baton Rouge. The Spanish settlers on the Gulf of 
Mexico, during the sixteenth and seventeenth centuries, 
extended their missions toward the north as far as Santa Fe, 
where the Rocky Mountains checked further progress in this 
direction. They therefore pushed westward to southern 
California. From there they followed the Pacific coast 
toward the north, establishing missions in the narrow area 
between the Coast Range and the Pacific. Their trail is 
now marked by cities still having Spanish names, such as 
San Antonio, Sante Fe, and along the coast, San Diego, 
Los Angeles, San Francisco, Sacramento, San Jose, and San 
Luis Obispo. 

The Berkshire Hills, in Massachusetts, exerted an impor¬ 
tant influence in settling the contest between Boston and 


PLATEAUS AND MOUNTAINS 


533 


and New York City for commercial supremacy. Freight 
brought from the West through the Mohawk Valley to Al¬ 
bany could be brought to New York by boat more cheaply 
than it could be hauled oyer the Berkshires by teams, and 
much of it was naturally deflected to New York. When rail¬ 
roads were built along the Hudson and through the Mohawk 
Valley, New York City acquired further advantage over 
Boston because of the Berkshires. Before a railroad line from 
Albany to Boston was completed, the position of New York as 
the chief seaport of the United States was fully established. 

Mountains are not absolute barriers. They are difficult to 
cross; but when sufficient incentive is provided, men always 
succeed in crossing them. 

In the case of the English colonists the necessary incentive 
came in the demand for more room and more virgin soil and 
in the increased importance of the trans-Appalachian fur 
trade. During the French and Indian War which followed, 
the possession of the best passes through the mountains was 
stubbornly contested, as is shown by the large number of 
battlefields between the Hudson and Lake Champlain, and 
between the Mohawk and Lake Ontario. 

The Rocky Mountains retarded the settlement of Cali¬ 
fornia more effectively than the Appalachians confined the 
colonists to the Atlantic coast, and for a longer period, be¬ 
cause of their greater height and breadth; but the necessary 
incentive came in the discovery of gold in 1848. Before the 
close of 1849, there were 100,000 people in California. 

Mountain Industries. — The excessive cost of transpor¬ 
tation in mountainous regions is so serious a handicap as to 
render manufacturing unprofitable when in competition with 
manufactories located on plains, unless the mountaineer can 
find his raw material on the mountain and can convert it into 
a light, finished product of greatly increased value. Thus 
lumber grown on the mountain is made into carvings and 
souvenirs and sold to tourists, thus entirely eliminating all 
cost of transportation. 


534 


NEW PHYSIOGRAPHY 


In mining regions, the handicap is less important than 
elsewhere, because the product is so valuable. The few 
dollars worth of gold obtained from a ton of ore can be carried 
down the mountain in one’s pocket. 

Mining. — One of the reasons why mining is so important 
among mountain industries is because igneous rocks are so 
often found in the center of the mountain mass as shown in 
the cross-section of the Rocky Mountains (Figure 296). 
Certain valuable ores are found in these deposits of igneous 
rock, others occur in rocks metamorphosed by contact with 
the igneous rocks, and still others are found in fissure veins 
or in porous rocks through which ground water circulated. 

A second reason for the importance of mining in mountain 
regions is the ease with which the kinds of rock forming the 
mountain is discovered. On plains, the kinds of rock under 
ground can be determined only by boring. On the mountain, 
erosion reveals the structure and the kinds of rock forming 
the mountain. In the United States we obtain a large per¬ 
centage of the supply of gold, silver, copper, iron, manganese, 
and anthracite coal from mountain mines. Much marble, 
slate, and granite are quarried in mountains. 

Bituminous coal, salt, rock phosphate, lead, zinc, and some 
iron ores occur in sedimentary rocks. These ores are mined 
in plains as well as mountains. 

Figure 302 shows the location of the gold and silver mines of 
the United States. Gold mines are found in the Appalachian Moun¬ 
tains, in the Black Hills, and in the western mountains. Silver 
mines are all located among the ranges of the Western Cordillera. 
Compare this map with Figure 279A to determine the position of the 
mines with respect to mountains. 

Can you suggest a reason for the large number of mines along the 
eastern boundary of California? 

Water Power. — The water power of mountain streams 
has long been utilized in cities along the fall line and by the 
miner in western mountains, but only a small percentage of 


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PLATEAUS AND MOUNTAINS 535 


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NEW PHYSIOGRAPHY 


mountain streams can be thus utilized. The possibility of 
electric transmission of power has greatly increased the value 
of these streams; and the public interest in the “ white coal,” 
as water power is called, speaks for its rapid development. It 
is destined soon to become a second important industry in the 
mountain regions and a great stimulus to our manufactures. 

The amount of power that can be obtained from a given 
fall depends upon the weight of water that flows over per 
minute and upon the vertical distance that the water falls. 
The product of these two numbers gives us the theoretic 
number of foot-pounds of work that the fall can produce, 
but this number can never be obtained since no water wheel 
is 100 percent efficient. 

If the flow of a stream were uniform throughout its course, 
its best water power would be where the highest fall was 
located. It is estimated that the total power that could be 
developed by the Mississippi River between St. Louis and 
New Orleans, where its slope is gentle, is only about 150,000 
horse power. A smaller quantity of water, flowing in the 
steep portion of the river near its source, could develop 
6,430,000 horse power. 

Our best water power opportunities are located in regions 
of rugged relief, generally at falls or steep rapids. There are 
numerous falls and rapids on mountain slopes, and the 
streams often flow in deep canyons that may be converted 
into great reservoirs for storing water until it is needed. 
These two advantages have led to an increase in the develop¬ 
ment of mountain power stations that leads to the hope that 
the millions of horse power now being wasted in mountain 
regions will soon be utilized. 

Irrigation. — The high mountains of the United States, 
like the Sierra Nevadas, the Cascades, and the Rockies, have 
heavy rainfall on their western slopes and an arid region east 
of them, because their courses lie across the path of the pre¬ 
vailing westerlies. 

The mountains are the cause of the arid region, but they 


PLATEAUS AND MOUNTAINS 537 

/ 

also enable us to restore fertility to the region through irri¬ 
gation, as artificial watering is called. Through their forests, 
glaciers, and lakes, mountains are great reservoirs of water 
which give rivers rising in mountains a much more uniform 
flow of water than that of rivers rising in plains. Then, too, 
mountain valleys of such streams can be converted into addi¬ 
tional reservoirs of great capacity by building dams across 
the valleys. From these reservoirs, canals carry water to each 
farm in the district. 

Agriculture. — Mountain slopes are not well adapted to 
agriculture, but there are regions where the number of in¬ 
habitants is so great that every foot of land must be made to 
yield its share of the food supply or some one will go hungry. 
In China, for example, steep slopes have been terraced by 
building stone walls at intervals on the slopes and by carrying 
earth up the mountain to fill the space on the upper side of 
the wall to produce a nearly level field. This requires labor 
that doubtless makes the fields thus built much more costly 
than the same area of level land; but when the work is done, 
the owner has a field that will furnish food as long as he lives. 
The Igorots in the Philippines also raise rice in such terraces. 
In Europe there are many terraced vineyards on mountain 
slopes. 

Terracing prevents erosion of the soil that so often destroys 
sloping fields as soon as the forests are cleared away. The 
roots of trees tend to hold the soil in place without terracing, 
and in some places mountain slopes have been slowly con¬ 
verted into orchards of apples, oranges, or olives by replacing 
the forest trees with fruit trees. In other sections there are 
groves of nut-bearing trees that yield as great a return per 
acre as our best wheat land. In all cases of mountain-side 
farming the cost of raising the crop and of transporting it to 
market is much greater than it would be if the land were 
level. The incentive that leads the farmer to incur the extra 
expense is usually the scarcity of level lands, but occasionally 
such conditions as the unusual fertility of a certain slope or its 


538 


NEW PHYSIOGRAPHY 


favorable exposure to the' sun's rays may lead him to culti¬ 
vate the hillside. 

t : Stock raising is an important industry in the western moun¬ 
tains of both North and South America; also in Switzerland, 
Germany, and Norway; and in Asia. It is about the only 
industry that can be carried on at elevations much above a 
mile; grasses, however, grow nearly up to the snow line, and 
herds are driven up the mountain side as spring climbs up¬ 
ward, to move down again -as autumn climbs downward. 
The land in the valleys near the lower limit of the stock-rais¬ 
ing belt are all used to supply hay for winter feeding. 

Forest Reserves. — The increasing population of the world 
demands greater and greater supplies of food, leads to the 
clearing of more and more of the level land each year, 
and drives the forests to the rugged lands. This increase 
threatens our supply of lumber and has already used up 
practically all of the timber that was growing on the plains 
of Europe. In the United States, we still have several large 
areas of level land, like the pine barrens of the coastal plains, 
that are forested because of their low fertility, but it is only 
a matter of time till these also will disappear. As soon as the 
demand for food is great enough, these areas will be cleared, 
fertilized, and turned to agricultural use. 

The United States Forest Service is now developing about 
200,000,000 acres of forest land belonging to the government, 
employing hundreds of men who patrol the reserves and 
enforce the rules of the service. It is its policy to prevent 
loss of trees through forest fires, through tree diseases, and 
through wasteful lumbering. 

It sees that all trees showing signs of deterioration are 
cut and sold for lumber or firewood, and that every tree that 
is cut is replaced by a young tree. This plan will eventually 
permit the sale of six or seven billion board feet of lumber 
annually without injury to the forests. 

It builds roads and maintains 1500 camp grounds that 
are open to the public. 


PLATEAUS AND MOUNTAINS 


539 


QUESTIONS 

1. Why is “Rocky Mountains” a more correct term than the 
name “Stony Mountains,” first given them? 

2. Show how a syncline may become a hill. 

3. Explain how the dropping of the wedge-shaped block in Fig¬ 
ure 289 indicates that the force acting there was a pull rather than 
a thrust. 

4. What kinds of rock form the summits of the ridges shown 
in Figure 295 (Appalachian Mountains)? Why? 

5. What do you think would- have been the effect upon the 
settlement of the United States if a continuous mountain range of 
great height had been where the Appalachians now are? 

6. Can you account for the presence of Arctic plants on the 
tops of isolated high mountains near the tropics? 

7. Why is the western slope of the Sierra Nevada Range forest 
covered, whereas the eastern slope resembles the Great Basin in 
barrenness? 

8. Mention several conditions which would tend to make the 
geographic cycle exceptionally long. 


CHAPTER XXIV 

VOLCANOES AND EARTHQUAKES 

Volcanoes have long been objects of interest to students of 
history and mythology, and much was written of the Mediter¬ 
ranean group, even by early Greek and Latin authors. This 
same region is also classic ground from a physiographic stand¬ 
point, because of the careful scientific study that has been 
made here of the phenomena of an eruption. 

Definitions. — A volcano is an opening in the earth through 
which lava and other heated materials are ejected. Some of 
the ejected materials pile up around the opening and form a 
cone of greater or less steepness, as the material forming it is 
coarse or fine, liquid or solid. In the top of the cone there is 
usually a cup-shaped depression called a crater. 

Causes of Volcanic Action. — Some geologists maintain 
that the heat comes from the interior of the earth; others 
that it is produced near the surface by chemical or mechani¬ 
cal means. A later theory of the origin of the heat maintains 
that it is due to the action of radioactive substances, like 
radium, which are known to be present in lavas. Whatever 
the origin of the heat, the fact remains that volcanoes are asso¬ 
ciated with young and growing mountains, and this suggests 
some relation between the uplifting forces and the origin of 
the heat. 

The force which causes explosive eruptions is undoubtedly 
steam pressure. All lavas contain water in greater or less 
quantity; and when they are deep down in the earth, the 
pressure keeps the water in the liquid state; but as the lava 
ascends, the pressure diminishes, until at last the water sud¬ 
denly becomes steam. If the lava is confined, the pressure 

540 


VOLCANOES AND EARTHQUAKES 


541 


increases as more and more water changes to steam; and 
finally the covering k rock bursts, just as a steam boiler does 
when the pressure becomes too great. If the lava is very 
fluid, and reaches the surface, the steam rises quietly, and 
oozing eruptions are likely to result. 

Phenomena of Eruptions. — The phenomena of an ordi¬ 
nary explosive eruption occur about as follows: A mighty 



Fig. 303 . — Fujiyama 
A young cone in Japan. 


explosion blows off the top of the cone, shatters the hardened 
lava, and sends steam, mingled with dust and ashes, high into 
the air, where it spreads out as a peculiar “ cauliflower cloud.” 
The falling stones and ashes destroy vegetation and may even 
bury whole cities. The rising steam, cooled by expansion and 
by mingling with the cold upper air, is condensed and falls as 
rain, accompanied by lightning. The rain brings down dust 
and ashes; and all together they form immense mud torrents , 
capable of burying cities, as for example, Herculaneum. 





542 


NEW PHYSIOGRAPHY 



Fig. 304. — A Cinder Cone in the Crater of Mount Vesuvius 
Photo by Publishers Photo Service. 

In volcanoes of the type of Vesuvius, after the explosion the 
liquid lava rises in the crater until it either overflows or, more 
frequently, until by its great pressure it rends the mountain 
and a lava flow escapes through the fissure thus formed. At 
first the lava may flow rapidly; but it gradually cools, 
hardens, slackens in speed, and finally stops. 



Fig. 305. — Cinder Cone 

Lassen Volcanic National Park, Upper California. Photo by M. E. Dittmar, 
Redding, California. 





Fig. 306 . — Mount Shasta, California 
Photo by Diller, TJ. S. G. S. 


VOLCANOES AND EARTHQUAKES 


543 




544 


NEW PHYSIOGRAPHY 


History of the Cone. — A volcanic cone, like all land forms, 
passes through a cycle of changes beginning with a period of 
rapid growth which is followed by a period of slow erosion 
that sometimes lasts until the cone is completely destroyed; 
but in many volcanoes, like Vesuvius, frequent eruptions 
interrupt the normal course of the cycle. 

Fujiyama , (Figure 303), the famous volcano of Japan, has 
a very perfect cone that shows only slight effects of erosion. 



Fig. 307. — San Francisco Peaks, Arizona 
Photo by Darton, U. S. G. S. 


A very young cone, recently formed on the floor of the 
crater of Vesuvius (Figure 304), is symmetrical in spite 
of the torrents of rain that have since fallen and that have 
caused the accumulation of thousands of tons of water in 
the crater. The youth of this cone is shown by the cloud 
of steam and smoke that still rises from its summit. 

The California cinder cone in Figure 305, although a mass 
of loose materials, shows little effect of erosion. The white 



VOLCANOES AND EARTHQUAKES 


545 



man has no record of the eruption that formed it, but it is 
probably the result of one of the recent volcanic eruptions in 
the United States. 

Mount Shasta, in northern California (Figure 306) is be¬ 
ginning to show the effects of both stream and glacial erosion; 
and the San Francisco Mountains of Arizona are in a more 
advanced stage (Figure 307). 

The volcanic neck (Figure 308) is a cylinder of basalt pro¬ 
jecting vertically from the ground and surrounded by frag- 


Fig. 308. — A Volcanic Neck, Adair, Colorado 
P hoto by U. 3. G. S. 

ments of the former cone in the throat of which the basalt 
cooled. A volcanic neck is the last remnant of the cone to 
disappear. Mount Royal, which gives its name to Montreal, 
is another example of a volcanic neck. 

Crater Lake, Oregon (Figure 244), illustrates another form 
assumed by some volcanoes in old age. The lake is between 
five and six miles in diameter, and the walls rise in places 
2200 feet above the water. It was formed by the sides of the 
former cone sinking into the crater and greatly enlarging it. 
Such craters are called calderas, a Spanish word meaning 
cauldron. The term is also applied to depressions formed by 











546 NEW PHYSIOGRAPHY 

an explosion that carries away the top of the volcano like that 
which destroyed Krakatoa in 1883. 

Columnar Structure. — The lava that cools slowly under 
great pressure contracts more or less symmetrically around a 
central core, breaking into columns. This characteristic 
columnar structure is exemplified in the Palisades of the Hud¬ 
son, in FingalPs Cave, Giant’s Causeway. (See Figure 309). 


Fig. 309. — Devil’s Post Pile 

Columnar structure in lava, head of San Joaquin River, California. U. S. G. S. 

Distribution of Volcanoes. — Active volcanoes are found 
where the crust is weakest; i.e , in or near the sea and in 
young and growing mountains. Most volcanoes lie in one 
of two belts. The best marked belt surrounds the Pacific 
Ocean. The other belt is an irregular one, passing through 
the Hawaiian Islands, the Mediterranean Sea region, and 
intersecting the first belt in the East Indies and in the West 
Indies. 

Products of Volcanic Eruptions. — At the instant that they 
are ejected, nearly all of the products are either in the liquid 
or the gaseous state, the only exception being the rock frag- 








Fig. 310 . — Distribution of Volcanoes 

O active. O dormant. • extinct. From Martoune’s Geographic Physique. Armand Colin, Paris. 


VOLCANOES AND EARTHQUAKES 


547 






























































































































































































































































































































































































































































































































































































































548 


NEW PHYSIOGRAPHY 


merits torn from the sides of the crater or blown from the 
overlying rocks. With the cooling of the products the steam 
condenses to water; and only the sulphur dioxide, carbon 
dioxide, hydrogen sulphide, chlorine, and related substances 
remain as gases. Lava that is free from bubbles of gas and 
solidifies quickly forms a solid mass like obsidian. 

When the lava is projected high into the air and solidifies 
before reaching the earth, it may be so filled with bubbles of 
the expanding gases contained in it that it becomes frothy 
or spongy and is called 'pumice. 

If the ejected materials cool before falling to the ground, 
they are known by various names, depending upon the size 
of the particles: volcanic dust, ashes, lapilli, and bombs. 
Materials of all sizes up to “ the size of an ox ” are ejected. 
Volcanic bombs, if their contained gases have expanded them 
as does bread in baking, are called bread cake bombs. 

Economic Products. — A Scotch firm purchased the cone 
of Vulcano, a small Mediterranean volcano, because of the 
alum, boracic acid, and sulphur that could be obtained 
from it. 

Pumice, sulphur , and borax are important volcanic products. 

Trap rock was used to pave the streets of Rome and the 
famous Appian Way; similar blocks from old volcanoes in 
Germany are floated down the Rhine to face the dikes of 
Holland, and from the Palisades came much of the material 
to pave the streets and parks of New York City. Volcanic 
dust and ashes when consolidated form tuff, a soft stone easy 
to work in the quarry, but hardening in air and becoming a 
very durable building stone, much used in Naples and Rome. 
Some of the oldest sewers in Rome, built of tuff 2500 years 
ago, are still in good condition. 

Volcanic dust and ashes exposed to plentiful rainfall rapidly 
weather and form a very fertile soil. Some of the finest 
orchards of New Jersey, some of the best farms of Oregon, 
and some of the most fruitful vineyards of Germany are on 
soils of volcanic origin. 


VOLCANOES AND EARTHQUAKES 


549 


Stromboli. — An irregular cone of volcanic material rises 
from the floor of the Mediterranean some 36 miles north of 
Sicily, to a height of about 6000 feet, one-half of which is 
above sea level. For more than 2000 years this volcano has 
been in a state of mild activity, emitting clouds of steam and 
showers of stones, and at night illuminating the cloud, which 
usually hangs over it, with flashes of light. This “ Light¬ 
house of the Mediterranean ” has guided sailors for centuries. 
Light, curling columns of steam rise from fissures in the crater 
at all times. At intervals, without the slightest warning, a 
sound is heard “ like that produced when a locomotive blows 
off steam/’ and a great volume of watery vapor, carrying 
many small masses of lava, is thrown violently into the air. 
The lava bombs are often in a semimolten condition when 
they fall to the earth. Such outbursts occur at frequent in¬ 
tervals. They are due to the escape of great bubbles of steam 
through the chilled and tenacious surface of the molten lava 
that fills the cracks. 

Etna. — The giant cone of Etna was known to the Romans 
as the “ Forge of Vulcan.” It is two miles high and about 
40 miles in diameter at its base. There are some 200 minor 
cones on its slopes. Its eruptions are preceded by earthquakes 
and loud explosions. Smoke, ashes, and cinders are dis¬ 
charged, and finally lava flows from the new cone formed. 
The large proportion of lava accounts for the gentle slope of 
the cone of Etna. 

Vesuvius. — The ancients knew Vesuvius as a mountain 
rather than as a volcano. At the beginning of the Christian 
era its crater, then about three miles in diameter, was 
covered with vegetation. Its slopes were cultivated, towns 
were located at its base, and there was no record of previous 
volcanic activity of the mountain. 

During the summer a.d. 79, a series of earthquakes of in¬ 
creasing severity occurred, and a new and strange cloud 
formed above its summit. Explosion after explosion occurred 
within the mountain; and the black cloud spread, shutting 



550 NEW PHYSIOGRAPHY 

out the light of the sun. Tacitus gives us two letters from 
the younger Pliny, who was an eyewitness of this eruption. 
One of these letters describes the experiences of his uncle, 
the elder Pliny, who lost his life near the foot of Vesuvius 


Fig. 311. — Explosive Eruption of Vesuvius in 1900 
The “Cauliflower cloud.” 

during an eruption. It seems that his party sought shelter 
from the shower of cinders and stones in a villa which “ shook 
from side to side ” from frequent earthquakes. When the 
accumulation of stones and ashes made it apparent that the 
villa would be buried, the party took to the fields, “ with 
pillows tied about their heads with napkins ” to protect them 
from the falling stones. 






VOLCANOES AND EARTHQUAKES 


551 


The second letter relates the younger Pliny’s experiences 
at Misenum, across the Bay of Naples from Vesuvius. He 
describes chariots standing on level ground without horses, 
which would not stand still even when the wheels were 
blocked with great stones, but “ kept running backward 
and forward ” with each earthquake. “ Besides this,” he 
continues, “ we saw the sea sucked down and, as it were, 



Fig. 312. — Eruption of Vesuvius in 1872 
Steam is rising from the crater and also from lava flows. 


driven back again by the earthquake.” Across the bay above 
Vesuvius “ was a dark and dreadful cloud, which was broken 
by zigzag and rapidly vibrating flashes of fire, and, yawning, 
showed long shapes of flame. These were like lightnings, only 
of greater extent. . . . Soon the cloud began to descend 
over the earth and cover the sea. . . . Ashes now fell, yet 
still in small amount. I looked back. A thick mist was close 
at our heels, which followed us, spreading over the country 
like an inundation. . . . Hardly had we sat down when 



552 


NEW PHYSIOGRAPHY 


night was upon us — not such a night as when there is no 
moon and clouds cover the sky, but such darkness as one 
finds in close-shut rooms. . . . Little by little it grew light 
again. We did not think it the light of day, but proof that 
fire was coming nearer. It was indeed fire, but it stopped 
afar off; and again a rain of ashes, abundant and heavy; 
and again we rose and shook them off, else we had been 
covered and even crushed by the weight. . . . Soon the 
real daylight appeared; the sun shone out, of a lurid hue, to 
be sure, as in an eclipse. The whole world which met our 
frightened eyes was transformed. It was covered with ashes 
white as snow.” 

No lava flow accompanied this eruption, but the enormous 
quantity of ash buried Pompeii and, mixed with rain, formed 
a mud stream which overwhelmed Herculaneum. There 
have been frequent eruptions of Vesuvius since this one, 
those of 1631 and 1906 being especially destructive. In these 
later eruptions the explosive action has been followed by 
lava flows. 

The eruptions of Vesuvius are unlike the mild, continuous 
action at Stromboli, and consist of paroxysms of great vio¬ 
lence separated by long intervals of quiet. During these 
intervals of rest the volcano is said to be dormant. 

Mont Pelee. — An eruption of this volcano on May 8, 
1902, destroyed the city of St. Pierre on the island of Marti¬ 
nique, one of the Lesser Antilles. Previous to this date it 
had been dormant for 50 years, but for days before the 
eruption it had shown signs of activity. Great columns of 
steam and ash were ejected, boiling mud flowed from the 
sides of the volcano, and repeated explosions occurred in its 
interior. Lightning flashed from the ascending cloud, and 
the frequent earthquakes broke all ocean cables leading to 
the island. 

On the morning of May 8, a dull red reflection was seen 
on the trade-wind cloud that covered the mountain summit. 
This became brighter and brighter, and soon red hot stones 



VOLCANOES AND EARTHQUAKES 553 

were ejected from the crater and bowled down the mountain 
sides, giving off glowing sparks. Suddenly a hot blast of 


Fig. 313 . — The Spine op Mt. Peu£e, Martinique, West Indies, 1902 
Courtesy of the American Museum of Natural History. Photo by Dr. E. O. Hovey. 

gases shot from the crater, and two minutes later engulfed 
the city of St. Pierre, five miles distant, in an atmosphere 
that was fatal to all who breathed it. It wiped out all 
vegetation and every living creature in its path. Buildings 



554 


NEW PHYSIOGRAPHY 


of the city and ships in the harbor instantly burst into flames; 
30,000 persons lost their lives. There was no lava flow, but 
the lava in the throat solidified and was forced upward by 
the pressure from below until it stood 1200 feet above the 
crater at its maximum. 

Krakatoa. — In 1883 the most violent explosive eruption 
of historic times occurred on the East Indian island of 
Krakatoa. The island was some 5 miles long and 3 miles 
wide, with an altitude of 2623 feet at its highest point. 

Nearly the whole of the lower part of the island and half 
of the highest peak were blown away. Dust was thrown into 
the air to a height of about 20 miles, and was carried to every 
part of the earth. Beautiful sunrise and sunset effects were 
caused for many months by this dust. The concussion of the 
explosion broke windows in Batavia, 100 miles away, and 
the report was heard 2267 miles. A mighty wave flooded 
the surrounding coasts to a depth of 50 feet, stranding ocean 
steamers, causing great loss of property, and drowning more 
than 36,000 people. For many weeks navigation was im¬ 
peded by floating pumice that covered the surface of the sea. 

Hawaiian Islands. — Hawaii is one of a group of islands 
with many volcanoes which in the main owe their existence 
to eruptions at the bottom of the ocean. This island is 
80 miles long and rises 30,000 feet above the ocean floor. 
There are four craters on the island, of which Mauna Loa is 
the highest. The eruptions of the volcanoes in the Hawaiian 
Islands are in sharp contrast with that of the island of 
Krakatoa. In these oozing eruptions there are no explosions, 
no showers of dust or ash; no great volume of steam is 
ejected, and earthquakes are rare. The lava flows (Figure 
314) sometimes continue for months, whereas eruptions of 
the explosive type last but a few days. Before an eruption 
the lava rises quietly in the crater until the great pressure 
fissures the side of the mountain, when a river of molten 
rock flows to the sea. The slopes of the volcano subject to 
oozing eruptions are very gentle, but this must not be under- 


VOLCANOES AND EARTHQUAKES 


555 



stood to mean that the cone is small. Mauna Loa is many 
times as large as Vesuvius, and its crater is a typical caldera, 
nearly 3 miles long, 2 miles wide, and 1000 feet deep. Ice¬ 
landic volcanoes are of this type. 

Active Volcanoes of North America. — Active volcanoes 
are numerous in Central America and Mexico, and some of 
the Alaskan volcanoes have been in eruption within a few 
years. 


Fig. 314 . — Hawaiian Lava Flow with Unbboken Surfaces 

Mount Katmai. — In June, 1912, an eruption of Mount 
Katmai, in Alaska, occurred, that was remarkable for the 
large amount of ash ejected. An area 15 miles wide on the 
south and west sides of the volcano was buried under 50 feet 
of ash, houses were damaged 100 miles away, the ash was 
noticeable more than 200 miles away, and total darkness 
prevailed for more than two days. The sound of the explo¬ 
sions was heard 750 miles down the coast. Fortunately the 
region about this volcano is sparsely inhabited. 

Lassen Peak. — In May, 1914, the first volcanic eruption 
in the United States proper, to be described by white men, 
occurred at Lassen Peak, California, which is in the 



556 


NEW PHYSIOGRAPHY 


Sacramento Valley, about 210 miles northeast of San 
Francisco. 

The eruption began with geyserlike jets or clouds of steam 
which augmented steadily until great bursts of smoke rising 
2000 feet formed a “ cauliflower cloud.’’ This was followed 
by pillars of fire visible 100 miles down the Sacramento 
Valley (Figure 315). 

The activity of Lassen Peak has continued intermittently 
to the present time. Sometimes the material ejected consists 
chiefly of gases, again cinders and bombs are hurled high 
in the air, and at other times lava breaks through the 
sides of the cone and runs quietly down the slope. 

The region about the peak is now a national park, con¬ 
taining many other evidences of the recent volcanic activity of 
the region, such as a lava flow estimated to have been ejected 
150 years ago, and the large cinder cone 800 feet high, 
believed to have been formed about 1700, shown in 
Figure 305. 

Recently Extinct Volcanoes of the United States. Mount 
Hood. — This mountain on the crest of the Cascade Range 
in Oregon is noted for its graceful outlines And for the fu- 
maroles and steaming rifts which still emit sulphurous fumes 
and indicate comparatively recent activity, although there 
has been no eruption within the memory of man. 

Mount Rainier. — This stately cone rises from near sea 
level to an altitude of 14,500 feet, and so appears much 
higher than most of those that reach a greater altitude. It 
has a bowl-shaped crater, below which on the sides of the 
mountain the rims of former craters may be seen. Jets of 
steam and gas still issue from small holes in its snow-clad 
summit, showing that its heat has not entirely disappeared. 

San Francisco Mountain. — This mountain in Arizona is 
much eroded, and no signs of a crater remain; but it is 
surrounded by lava flows and beds of cinders; and several 
hundred cinder cones, formed by volcanic eruptions, are 
found in the immediate vicinity. Some of these cones were 


VOLCANOES AND EARTHQUAKES 


557 



Fig. 315. — Lassen Peak, California 


The United States has one active volcanic peak, Lassen Peak, in Lassen County, 
California. It is located about 210 miles northeast of San Francisco. Intermittently 
since 1914 this peak has passed through the various stages of activity, at times simply 
belching forth gases, while at other times it has cast rock fragments and lava to great 
heights. The picture was made at the exact moment of an explosion. Towering 10.437 
feet above the sea, Lassen Peak is a conspicuous feature of the landscape of the Sacra¬ 
mento Valley. Photo by Underwood and Underwood. 



558 


NEW PHYSIOGRAPHY 



Fig. 316 . — Mount Rainier, near Tacoma, Washington 
A dormant, dome-shaped volcanic cone. 




VOLCANOES AND EARTHQUAKES 


559 


formed so recently that erosion has not modified the original 
form of the cone (Figure 307). 

Mount Taylor. — On one of the large mesas of western 
New Mexico, Mount Taylor, rises to an altitude of 11,000 
feet. The mountain is almost entirely composed of lava, 
and the mesa is covered by a cap of lava. This cone is also 
much eroded. In the lowland about the mesa are many 
volcanic necks, each one a mass of lava which cooled in the 
throat of a volcano that has disappeared. 

Mount Shasta. — This extinct volcano of northern Cali¬ 
fornia is in some respects like Etna. It towers 11,000 feet 
above a base 17 miles in diameter, is snow clad even in 
summer; and its eruptions were explosive, followed by great 
lava flows. There are two great craters, the younger being 
near the top of one side of the older cone. Some 20 smaller 
cones are found near the base of the mountain, and from 
one of these a lava flow may be followed more than 50 miles. 
The cone is much dissected by glaciers and streams, but is 
still in its youth (Figure 306). 

Other Indications of Volcanic Activity. — The Columbian 
lava plateau covers a large part of Washington, Oregon, and 
Idaho with successive layers of lava, which in places reach 
a total thickness of 5000 feet. The section of this plateau 
suggests stratified rock, but the layers represent distinct 
flows of lava, which are sometimes separated from the next by 
layers of soil in which the roots and trunks of large trees are 
preserved. This proves that a long interval of time elapsed 
between the flows. Because of the absence of cones in this 
region, it is thought that the lava came through fissures. 
The surface is covered with residual soil of great fertility. 
This plateau is cut by many deep canyons in which the 
structure of the plateau is shown. 

Besides the masses of lava that were evidently poured 
out on the surface of the land, like those that formed the 
Columbian Plateau, there are many others that indicate 
former volcanic activity although they did not reach the 


560 


NEW PHYSIOGRAPHY 


surface of the earth. In Figure 317 the solid black masses 
represent lava that was forced up from below. 

The left-hand figure, /, is a laccolith (the word meaning 
stone lake). The lava was forced upward by enormous 
vertical pressure that raised the overlying strata forming a 
dome. An interesting feature of the laccolith is the fact 
that the great pressure was not transmitted in all directions 
in accordance with Paschal’s Law, which would have 
caused the lava to spread horizontally. The lava acts like 
a solid in this respect and was probably in a pasty condition. 



Fig. 317. — I, laccolite; IT, intrusions with Dikes; III, extrusion with dike; 
IV, vent for Ashes, etc. 

Penck. 


The second figure is a mass of intrusive lava that spread 
horizontally without doming the strata above it. Such 
sheets are called sills. 

The Palisades of the Hudson is a sill 30 miles long and 
several hundred feet thick, and the Watchung Mountains 
of New Jersey as well as the lava sheets of the Connecticut 
Valley are other illustrations. The third figure is a dike 
leading to the surface where it formed an extrusive sheet 
like the Columbian Plateau. A dike is a mass of lava that 
solidified in a vertical or nearly vertical fissure. 

Aside from sills and dikes, there are few indications of 
volcanic activity east of the Rocky Mountains. Erosion 
has destroyed such volcanic cones as may have existed, 
but it has exposed numerous sheets of lava which were in¬ 
truded between the layers of sedimentary rocks. The Pali¬ 
sades of the Hudson, the lava sheets of the Connecticut 













VOLCANOES AND EARTHQUAKES 


561 


Valley, and the Watchung Mountains of New Jersey are 
examples of such sheets. 


Earthquakes 

Definition. — The lithosphere is constantly in a state of 
tremor. Sometimes the vibrations are so slight as to pass 
unnoticed except as recorded by the most sensitive instru¬ 
ments and at other times so violent as to destroy whole cities. 
Some of the earth tremors are due to human activities, such 
as the movement of railway trains or the explosion of 
dynamite; others are due to natural causes. 

The vibrations travel through the lithosphere and may be 
detected at greater or less distances from their source, as the 
violence of the shock which causes them is greater or less. 

An earthquake is a tremor of a part of the lithosphere pro¬ 
duced by natural causes. 

The Ischian Earthquake. — On July 24, 1883, the island 
of Ischia, near Naples, Italy, was shaken by an earthquake 
which was not preceded by warning shocks and which lasted 
but 15 seconds. Violent detonations accompanied the 
tremors, 1200 houses were destroyed, 2300 persons were 
killed, fissures were opened, and landslips occurred. Survi¬ 
vors tell us that the whole town seemed “ to jump into the 
air ” and fall in ruins. 

On this island is the great crater of Epomeo, which was in 
eruption in 1302, after at least 1000 years of slumber. No 
eruption of Epomeo accompanied this earthquake, but it is 
believed that the underground explosions which caused the 
earthquake were of volcanic origin and indicate future ac¬ 
tivity of Epomeo. 

The Charleston Earthquake. — During the last week of 
August, 1886, slight earthquake shocks occurred at intervals 
at Charleston, South Carolina. Their violence gradually in¬ 
creased and culminated at 10 p.m., August 31, in one of the 
great earthquakes of the last century. There was first no- 


562 


NEW PHYSIOGRAPHY 



ticed a distant rumble which increased in intensity as though 
an enormous railway train was approaching through a tunnel 
beneath the city. As this rumble became a roar, the ground 
seemed to rise and fall in visible waves. The disturbance 
lasted about 70 seconds and was repeated with equal violence 
eight minutes later. 

During these tremors men could not stand, chimneys were 
thrown down, and every building in the city was damaged. 


Pig. 318. — The Fault Trace, San Francisco Earthquake 

Great cracks were opened in the earth, and both underground 
and surface drainage were disturbed; railroad tracks were 
twisted and bent, and 27 persons were killed. The shock was 
felt as far north as Canada. 

This earthquake is notable for the information concerning 
earthquakes derived from the study of its phenomena. The 
location of the origin of the disturbance was determined, and 




VOLCANOES AND EARTHQUAKES 


563 



Fig. 319. — Ruin of the $7,000,000 City Fall by the San Francisco Earthquake 

and Fire 





Fig. 320. _Mission Street, San Francisco, after the Earthquake 






























564 


NEW PHYSIOGRAPHY * 


the velocity with which the earthquake wave traveled in this 
case was shown to be 150 miles per minute. 

The earthquake was succeeded by several less severe shocks 
during the night, and slight shocks were observed in the 
region for several months. 

The San Francisco Earthquake. — About 5 a.m., April 18, 
1906, an earthquake occurred on the California coast which 
lasted 67 seconds. During this short interval many buildings 
in San Francisco were wrecked and the water supply was cut 
off, so that the fire which followed destroyed a large part of 
the city. Figures 319 and 320. Many landslides occurred at 
the same instant in the mountains of the district affected, 
cracks were opened in the earth, and some regions settled 
several feet. 

The earthquake was due to slipping along an old fault 
plane, which has been traced nearly 400 miles (Figure 318). 
The average vertical displacement was slight, but the hori¬ 
zontal displacement was in places as much as 20 feet. 

Messina Earthquake. — At 5.23 a.m., December 28, 1908, 
the region about the Strait of Messina, in Southern Italy, 
experienced one of the most disastrous earthquakes in the 
history of the world. The cities of Messina and Reggio were 
reduced to a shapeless mass of ruins, several smaller towns 
were more or less damaged, and upwards of 200,000 persons 
were instantly killed or imprisoned in the ruins, so that rescue 
was impossible. 

The ground seems to have been suddenly raised and then 
dropped, causing the buildings to collapse; great fissures 
opened; the wharf sank to the level of the sea,and a sea wave 
from six to ten feet high swept over the lower portions of the 
region. 

The earthquake was preceded by several slight shocks, and 
the seismic activity continued for several weeks. 

Extensive breaking of telegraph cables in the vicinity in¬ 
dicates a submarine disturbance, and the center of the dis¬ 
turbance was a line through the Strait of Messina. These 


VOLCANOES AND EARTHQUAKES 


565 


two facts make it probable that the earthquake was due to 
slipping along the old fault plane which runs through the 
strait. This fault plane has probably been the seat of many 
earthquakes. The total vertical displacement of one side 
of this old fault is known to be several thousand feet. 

Distribution of Earthquakes. — No portion of the earth 
is entirely free from earthquakes, although most of them 
occur either in the vicinity of active volcanoes or near grow¬ 
ing mountains. It will be observed that the borders of the 
Pacific Ocean are particularly subject to earthquakes and 
that a belt of seismic activity crosses Eurasia, beginning at 
Gibraltar and following the general direction of the Mediter¬ 
ranean Sea. 

There have been in recent times relatively few earthquakes 
among the older mountains which border our eastern coast. 
Professor Shaler has called attention to the fact that in New 
England there can have been no violent earthquake since 
glacial times, for such an earthquake would have displaced 
the numerous balanced rocks to be found there. 

Cause of Earthquakes. — All great earthquakes are vibra¬ 
tions established by the sudden yielding of the earth’s crust 
(the lithosphere) to the stresses set up within it by lateral pres¬ 
sure. Such earthquakes are steps in the natural process by 
which our plateaus and mountains have been uplifted. All 
of the earthquakes described above, except the Ischian, were 
of this class. In some instances the strains due to lateral 
pressure are relieved by fracture of the lithosphere, which is 
accompanied by faulting, and at others by lateral displace¬ 
ment of one side of the fissure, without faulting. In some 
instances the strains are relieved by slipping along an old 
fault plane. 

Many earthquakes are caused by explosions which accom¬ 
pany volcanic eruptions. The Ischian earthquake was proba¬ 
bly of this type, and like others of its cl^ss was a minor 
disaster. 

Any natural phenomenon which results in a heavy blow to 


566 


NEW PHYSIOGRAPHY 





Fig. 321. — Earthquake Regions 

After Monte.ssue de Ballore, Tremblements de la Terre, Armand Colin, Paris. 








































































































































































































































































































































































































































































































VOLCANOES AND EARTHQUAKES 567 

the lithosphere might throw a portion of it into a vibration 
which would be classed as an earthquake, if the action oc¬ 
curred below the surface. Slight tremors may have been 
caused by great landslides, by the fall of the roof of a large 
cave, or by similar accidents. 

Sea Waves. — When an earthquake occurs near the sea, 
great sea waves often increase the disaster. The water at first 
recedes from the land, sometimes leaving vessels stranded on 
the exposed sea bottom. Then a great wave advances, which 
has in some instances swept the vessels over the tops of houses 
and has stranded them far inland. In the earthquake at 
Lisbon, Portugal, in 1755, some 30,000 people who had sought 
safety on the wharves were drowned by the sea wave. These 
waves have been called tidal waves , an obvious misnomer. 


QUESTIONS 

1. Criticize the definition, “A volcano is a burning mountain, 
belching forth fire, smoke, and lava.” 

2. How can people near a volcano tell that an eruption is proba¬ 
bly about to take place? 

3. Contrast eruptions of Vesuvius, Pelee, and Hawaiian vol¬ 
canoes. 

4. Give in their natural sequence the phenomena of an ordinary 
explosive eruption. 

5. State and account for the distribution of volcanoes. 

6. Which is the most valuable volcanic product? Why? 

7. What quakings of the earth are not properly called earth¬ 
quakes? Why? 

8. Contrast the two great classes of earthquake phenomena. 

9. Define seismic and seismograph. 

10. State and account for the general distribution of earthquake 
regions. 

11. After an earthquake why does the water first recede near the 
shore? 

12. Where would there be more danger of earthquakes — along 
a mountainous or along a coastal plain shore? Why? 


CHAPTER XXV 


SHORE LINES AND HARBORS 

Definitions. — The shore line is the line along which land 
and water meet. The shore is the margin of land next to any 
large body of water, whereas the coast is the margin of land 
next to the sea. The beach is that portion of the shore that 
lies between high and low water levels. The continental shelf 
is the submerged portion of the continental mass adjoining 
the shore and extending seawards with gentle slopes. The 
depth of about 600 feet is generally taken as the outer limit 
of the shelf. 

A curious distinction in the usage of these terms is quite 
common; we often hear inhabitants of the interior of the 
country speak of “ going to the coast,” but you never hear a 
sailor speak of “ going acoast ” or of having “ coast leave,” 
he always uses the terms ashore and shore leave. But when 
a sailor speaks of a ship that sails along a coast in search of 
trade, he calls the ship a coaster and the act coasting. As 
occasion requires a sailor will also speak of the Coasting Act, 
the coast pilot, the coast charts, and the Coast Guards. 
When our inland friends speak of going to the shore, they 
usually mean that they are going to the beach. 

Regular Shore Lines. — The rock waste brought to the sea 
by streams or cut from the cliffs settles to the bottom as the 
water loses its velocity, filling hollows and tending to make 
the bottom of the sea more and more level as time goes on. 
Continued deposition in a given locality will eventually bring 
this waste within the reach of waves and longshore currents, 
which will soon spread it out as a smooth surface that slopes 
gently downward away from the shore. This deposit is the 

568 


SHORE LINES AND HARBORS 


569 


continental shelf. When enough sediment accumulates so that 
a beach is formed, the shore line will be regular. 

Emerged Shores. — The time necessary to form a regular 
shore line in this way will be shortened materially if the 
region where such deposits are forming emerges from the sea 
so as to locate the shore line on the offshore deposits. If the 
shore line is not quite regular at first, it would be smoothed 
quickly. Very perfect examples of regular shore lines have 



Fig. 322. — A Regular Shore Line Due to Recent Emergence of the Land, 
Nome, Alaska 


been formed by this sort of “ seaward migration ” of the 
shore line. 

Figure 322, traced from the special topographic sheet of the region 
about Nome, Alaska, shows a portion of a narrow continental shelf 
that recently emerged, producing a very regular shore line. It is 
interesting to note that near Nome, there are peaks of several 
small extinct volcanoes not yet covered by the deposits of the con¬ 
tinental shelf and that the peaks now form shoals near the shore. 

Wave-cut Shores. — Many regular shores have been 
formed by wave erosion which causes a landward migration 
of the shore line without necessary elevation or depression 
of the land mass. 





570 


NEW PHYSIOGRAPHY 


Figure 323 is a tracing of a part of the Wellfleet, Massa¬ 
chusetts, topographic sheet, and Figure 326 is based upon 
the information given on the coast chart of the region. 

The shore line here is as regular as that at Nome, but the 
material that forms the beach was wrested by wave erosion 
from the cliffs of glacial material that form the back of Cape 
Cod. The shore line migrated landward as it was formed 



Wellfleet, Massachusetts. 


instead of seaward as it did when the Nome, Alaska, shore 
was formed. 

The section (Figure 326) is located where the towers of 
the Marconi Wireless Company were erected. This was the 
first wireless station built in this country for transatlantic 
communication. While it was under construction, the resi¬ 
dents of the region notified the builders that the shore at the 
point selected was being worn away and that the towers 
would soon fall into the sea. The warning was unheeded, and 
the station was completed. It remained long enough to make 
its contribution to the development of our present-day radio, 
but the towers are now down and the station is abandoned. 










SHORE LINES AND HARBORS 


571 


Does the fact that the upper surface of the cliff slopes downward 
from A to B indicate that at least half of the ridge that originally 
formed Cape Cod has been worn away? 


There is another wave-cut beach just south of the entrance 
to New York Harbor, extending from Monmouth Beach to 
Bayhead, New Jersey (Figure 
324). The cliffs here are 
neither so high nor so steep as 
those on the back of Cape 
Cod. 

Compare the sections Fig¬ 
ures 325 and 326. The scales 
of the two sections are equal. 

Note the difference in the 
height of the cliffs and the 
fact that in each case the slope 
is steeper near the shore than 
elsewhere. 

If wave erosion were as 
active at a depth of 20 
feet as it is at the surface, 
the shore would be eroded as 
fast at that depth as at the 
surface, and the shore would 
have a nearly vertical drop 
of 20 feet. In other words 
the decreasing slope of the 
bottom indicates decreasing activity 
the depth of the water increases. 

The effectiveness of wave erosion at different depths is 
also shown by the depth lines plotted offshore in Figure 324. 
The three-fathom curve is almost exactly parallel to the 
shore line, the 30-foot curve is less regular than the 18-foot, 
and the 50-foot curve is much more irregular than either of 
the others. What does this indicate as to the level at which 



Fig. 324. — Shore Line from Deal 
Beach, New Jersey, to Sea Girt, 
New Jersey, from the Asbury Park 
Sheet 

U. S. G. S. and coast charts. 

of wave erosion as 









572 


NEW PHYSIOGRAPHY 


the smoothing action of the waves is most effective? What 
effect does the rise and fall of the tides have upon the loca¬ 
tion of the smoothest part of the shore? 

Suppose that this coast should emerge so that the shore 



Fig. 325. Section of the Cliff and Beach at Allenhurst, New Jersey 
From Asbury Park sheet and coast charts. 


line coincided with the 50-foot curve. Would you call the 
shore regular or irregular? 

What effect would the waves have on its regularity? 



Marconi station, Wellfleet, Massachusetts. 

What length of time would the waves require to effect the 
change — a few years or a thousand years? 

An approximate estimate of the distance that the shore 
line in Figure 324 has migrated westward because of wave 
erosion may be made by comparing the distance from the 











SHORE LINES AND HARBORS 573 

shore line to the average position of the 50-foot contour line 
(shaded) between the streams , with a similar distance measured 
on Figure 329. This gives us about two and one-third miles 
on Figure 324 and about three and a quarter miles on Figure 
325, indicating that the waves have eroded a strip nearly a 
mile wide from Monmouth Beach to Bayhead. 

Outwash Plane Shores. — Regular shore lines have been 
formed in a number of other ways besides the seaward mi¬ 
gration of the shore line; for example, when the waves of the 
ocean or a lake act upon the unconsolidated material of an 



Fig. 327. — Formation of a Bay-Mouth Bar 


outwash or an alluvial plane, as it did at Easthampton, 
Long Island, and Dennisport, Massachusetts. 

Constructive Work along Shore. — The shore line in Fig¬ 
ures 323 and 324 are the result of destructive action of the 
longshore forces. We shall consider now some of the 
regular shores constructed by these forces through trans¬ 
portation and deposition by water, already studied in 
Chapter XXI. 

Bay-mouth Bars and Spits. — Wherever there is a sharp 
bend in the shore line because of a bay CD (Figure 327), 
shore currents will spread out into the bay when the tide 
is rising and, meeting quiet water behind the point C, will 
deposit any sediment that they are transporting. This ac¬ 
tion will result in the growth of a spit from C when the cur¬ 
rents have the direction shown by the arrow, or from the 
point D when the current is reversed. If the shore is of 







574 


NEW PHYSIOGRAPHY 


unconsolidated material or if there is an abundant supply of 
sediment received from a river or cut from some nearby cliff, 
the growth of such bay-mouth bars may be very rapid. Rock- 
away Beach (see map of New York Harbor, Figure 338) has 
increased in length about six miles since the Coast Guard 
station was first established on the point. 

When such bars are attached to the mainland at one end 
only, they are called land spits, because of their resemblance 
to the spits on which our ancestors used to roast meats 



Fig. 328. — Bay-Mouth Bars, Martha’s Vineyard, Massachusetts 


before an open fire. Rockaway Point is a spit, and so is 
Island Beach shown in Figure 329. 

Offshore Bars. — Both Rockaway Point (Figure 338) and 
Island Beach (Figure 329) are separated from the mainland 
shore line by a lagoon. 

South of Island Beach we have many such offshore bars 
that are really islands and form a row, some miles off the 
mainland coast, extending to Texas. The seaward shore of 
these bars, sand reefs , or barrier beaches, as they are variously 
called, is exceptionally regular and may be taken as the type 
of regular shore lines. Along this shore is a series of wonder¬ 
ful beaches, unexcelled for bathing; but examination shows 
that after a storm they are frequently strewn with wrecks, 








Fig. 329. — The New Jersey Shore Line from Bayhead to Barnegat 
Reduced from the Barnegat sheet, U. S. G. S., and the coast chart. 


SHORE LINES AND HARBORS 


575 


















576 


NEW PHYSIOGRAPHY 


and that here and there the timbers of wrecks of previous 
storms protrude from the sand. 

Consulting the map, we find that every mile or so there is 
a United States Coast Guard Station manned by brave men 
whose duty it is to rescue shipwrecked sailors. Lighthouses 
are far apart, and except for the magnificent beacon on the 
highlands of Navesink, proclaiming the proximity of New 
York Harbor, most of them mark the location of shallow 
harbors that only small vessels can ent£r. 

Between New York Harbor and Cape May, a distance of 
about 130 miles, large vessels must put to sea when a storm 
approaches and ride out the storm, because there is no harbor 
near them in which they can seek safety. More disasters 
have occurred on this coast than on any other of equal extent 
in the United States. Such shores are broken only where 
some stream empties or where a large body of water behind 
the bar maintains a tidal inlet. 

In Yucatan, on the west coast of southern Africa, and on 
many other regular coasts the absence of good harbors obliges 
ships that wish to land passengers or freight to anchor out 
beyond the line of breakers and transfer passengers and 
freight to small boats. Only the construction of artificial 
harbors can prevent the necessity of this transfer. 

Fault-plane Shores. — In many parts of the world a fault 
in the bedrock forms the shore. The seaward side of the 
fault is submerged, and the landward side forms a long row 
of rocky cliffs that form a very regular shore line. Harbors 
on such a coast are likely to be small, shallow, and far apart. 
Such shores make commerce even more difficult than the 
offshore bar shore line does, because there is the added diffi¬ 
culty of moving freight up and down the face of the cliffs. 

Figure 330 is a block diagram of such a coast. The fault plane, 
or the plane of the break, is marked 1, 2, 3, 4. The unshaded por¬ 
tion is water standing at the level 5, 6, 7, 8, and submerging one 
side of the fault. The surface of the fault plane, marked 2, 3, 

6, 5, forms the row of cliffs above water. 


SHORE LINES AND HARBORS 


577 


Beaches and continental shelves on such fault-plane coasts 
are rare except on those that are approaching old age, but 
they will be built in time as the cliffs are weathered and 
eroded and as sediment brought in by streams accumulates. 
Drowned valleys are absent. Fault-plane shores occur in 



Fig. 330. — Diagram of a Fault Plane Shore 
After Cotton. 


New Zealand, on the shores of the Red Sea, and on the west 
coast of Africa. 

Mountainous Shores. — Young folded mountains parallel 
to and near a shore would form a very regular shore line if 
they had emerged enough to raise the passes well above the 
sea. The western coast of the United States has the coast 
range quite near the shore and the shore line resembles the 
smooth curves of the offshore bars much more than it does 





578 


NEW PHYSIOGRAPHY 


the irregular shore of Maine. It is also like the regular 
shores in the scarcity of good harbors. Figure 331 shows 
about 125 miles of the California coast without an important 
harbor. There are but two large rivers that have eroded a 
channel through the western mountains and formed an im¬ 
portant harbor on the western coast of the United States. 
They are the Columbia and the Sacramento. The scarcity 
of good harbors is a characteristic of many mountainous 
coasts. 


Irregular Shore Lines 

• 

Submerged Coastal Plain. — When the area between the 
fall line and the seaward limit of the continental shelf was 
again partly submerged (Figure 227), 
after its many centuries of weathering 
and stream erosion, sea water drowned 
the deep valleys, giving us the irregu¬ 
lar shore line shown on the mainland 
side of the lagoon in Figure 329. 

This shore is low and marshy, but 
the land rises gently to the fall line. 
The strata beneath the soil are the 
same as those of the continental shelf 
and contain many marine fossils which 
prove the former submergence of the 
region. 

Figure 329 was reduced from the 
Barnegat sheet, United States Geo¬ 
logical Survey. It shows the general 
character of the mainland shore line 
of eastern North America south of 
Bayhead. It is irregular, without the 
advantage of numerous harbors that we expect irregular 
shores to afford. This is because the offshore bars and 
shallow lagoons are in front of all these minor irregularities. 
The major irregularities of the coast, such as New York 



Fig. 331. — Shore Line 
in California 





SHORE LINES AND HARBORS 


579 


Bay (Figure 338) Delaware Bay, Chesapeake Bay, and sev¬ 
eral others, are among the most important harbors of eastern 
United States. They were eroded by large rivers that cut 
through the present Appalachian ridges. The 50-foot con¬ 
tour line on the mainland is shaded to show the nature of 
the new shore line that would be produced if the present 
coast should be submerged an additional 50 feet. Notice 
that the new shore would be more irregular than the pres¬ 
ent shore, because of the great amount of erosion that has 
occurred. The magnitude of the erosive work accom¬ 
plished suggests that the time required must have been 
exceedingly long. 

One peculiar feature of the new shore line shown by the 
50-foot contour is offshore islands of a nature quite dif¬ 
ferent from the present ones. These irregular-shaped off¬ 
shore islands are a characteristic feature of most irregular 
shores. Before we leave Figure 329, notice the evidence of a 
submarine delta on the seaward side of Barnegat inlet. How 
is it indicated and what motion of the water formed it? 

Can you point out two indications that there was once an 
inlet at Ortley near the Toms River? 

The valleys drowned on the eastern coast of the United 
States were eroded, as a rule, in unconsolidated material 
which accounts for their U-shaped cross-section and the gen¬ 
eral narrowing of the submerged portion as we follow them 
inland. The drowned valley of the Hudson River, above the 
Harlem, was eroded in bedrock by stream and glacier. The 
lower part of its valley is neither^ U-shaped nor funnel- 
shaped. 

The Ria Coast of Spain. — On the northwest coast of 
Spain, between Portugal’s northern boundary and Corme, 
a number of low and nearly parallel mountain ranges, whose 
trend is about at right angles to the coast, slope down to the 
sea and disappear beneath its surface. 

The valleys between these ranges are submerged and form 
funnel-shaped bays or rias that extend from ten to twenty- 


580' NEW PHYSIOGRAPHY 

five miles inland. They have wide entrances, (Figure 332) 
and their appearance on the map reminds one of Chesapeake 
Bay, though the actual appearance of the coast is very unlike 
that of a coastal plain. The shores are rocky with many 
high cliffs and present a sharp contrast with the low and 

marshy shores of the 
Atlantic coastal plain. 
These bays make ex¬ 
cellent harbors, that of 
Ferrol being the chief 
naval harbor of Spain. 
The submerged valleys 
doubtless owe their shape 
in some degree to erosion; 
but glacial erosion played 
no part, for this is an 
unglaciated region. 

The Maine Coast. — 
The air-line distance from 
Portland, Maine, to New 
Brunswick, according to 
Professor Cleland, is 200 
miles, whereas the shore 
line between these points 
is 2000 miles. Figure 
333 shows a part of the 
Maine coast. There are many offshore islands and rocky 
promontories and many estuaries , as wide-mouthed rivers 
subject to tides are called. Some of these estuaries are sub¬ 
ject to tidal ebb and flow for 25 miles. 

The islands are rocky, but there is much good farm land. 
Beaches are few, safe harbors are numerous, and lighthouses 
are abundant; but there are no coast-guard stations, because 
wrecks are comparatively few. The time after the warning 
of the approach of a storm is sufficient to enable the vessel 
to anchor in a safe haven before the storm breaks. 









SHORE LINES AND HARBORS 


581 



Boothbay topographic sheet, U. S. G. S. 











582 


NEW PHYSIOGRAPHY 


Figure 333 shows that the region has been much eroded. 
The precise extent to which streams are responsible for it, 
however, is difficult to determine, because so many of the 
evidences of stream erosion have been obliterated by later 

glacial erosion. Among the 
many evidences of glacial 
erosion in the Maine region, 
we will mention only the 
smoothed and striated surface 
of the bedrock and the 
U-shaped valleys , cross-sec¬ 
tions of two of which are 
shown in Figure 334 A and B. 

The frequency of “ rock- 
bottom ” and “ hard-bottom ” 
soundings ranged in lines off 
the larger islands of this 
coast indicates that sedi¬ 
ment has not yet accumulated 
in sufficient quantity to form a smooth continental shelf. 

A conclusion confirmed by the depth curves shown in Figure 
330 also shows that a seaward migration to the location of 
the 100-foot depth curve would not produce a regular 
shore line. 

The 100-foot contour line is shaded as the 50-foot contour 
was in Figures 324 and 329. This is done to show what kind 
of shore line we would have if the region should be depressed 
100 feet, in which case the new shore line would coincide 
with the 100-foot contour. 

Fiord Shore Lines. — In high latitudes, where glacial 
ice of enormous thickness and therefore great eroding power 
accumulates, glacial troughs of great depth are formed 
which sometimes extend far below sea level. When the 
glacial ice recedes, the lower ends of such troughs will be 
submerged, forming fiords. 

Fiords are narrow, branching bays of great depth, with U- 



Fig. 334. — A, Cross-Section of Line- 
kin Bay at Fisherman Island; B, 
Cross-Section of Sheepscot Bay at 
Lower Mark Island 






Fig. 335. — A Norwegian Fiord 


SHORE LINES AND HARBORS 


583 




584 


NEW PHYSIOGRAPHY 


shaped cross-section, and precipitous sides. They often have 
a smooth and quite level rocky surface on either side of 
them. Figure 335 shows a typical fiord. 

Folded Mountains Parallel to Shore. — When such moun¬ 
tains are submerged so that the passes are drowned, the 
valleys back of them will also be drowned, forming an 
irregular shore line which sometimes has long narrow islands 



The east coast of the Adriatic south of Istria. The ranges run parallel with the shore. 

like those on the east coast of the Adriatic shown in Figure 
336. 

If the passes are close together, separate peaks may 
form islands like those on the northwest coast of North 
America, north of 50° north latitude, and in southern Chile. 

Coral Reefs. — Southern Florida, the Hawaiian Islands, 
and the shores of all oceans of the torrid zone, except the 
eastern shores of the Atlantic and the Pacific, are fringed 
with jagged coral reefs. 

The reef-building coral is a small animal living in colonies 
attached to the ocean floor. It requires clear, warm, salt 
water currents to bring it food and light which it cannot get 










SHORE LINES AND HARBORS 


585 


much below a depth of 120 feet. It extracts limestone from 
sea water and deposits it in the lower part of its body. By 
the growth and decay of countless corals, the rocky base may 
be built up nearly to the surface of the sea. The waves 
break off branches of the coral and grind them to coral sand 
which finally consolidates to a granular limestone. The 
waves and the wind may build up a low reef, not more than 
20 feet above the level of the sea. 

Where the reef is close to the shore, as along eastern 
equatorial Africa, Brazil, Cuba, and the Hawaiian Islands, 
it is called a fringing reef. The outer border, better supplied 
with food, grows more rapidly than within. Within, the 
corals die, and the rock is dissolved until a lagoon may 
develop inside the barrier reef, as it is now called. The 
Great Barrier Reef of the northeast coast of Australia is 
about 1200 miles long. 

An atoll, or ring of coral around a central lagoon, may be 
formed where the coral has grown on the top of a shoal that 
came to within 120 feet of the surface. Or according to 
Darwin’s theory, the atoll is a coral reef around a sunken 
island, as for example a volcano. The growth of the coral 
equaled the rate of sinking. When the rate of sinking, 
or rise of water, exceeds the rate of growth, the coral are 
drowned. The Chagos Islands in the Indian Ocean are the 
unsubmerged portions of a very extensive coral region. 

Coral islands are naturally very low, though some few 
show that they have been elevated. Plant life may be abun¬ 
dant, though of few varieties. The cocoanut palm fur¬ 
nishes food, clothing, and utensils to the unambitious 
natives. 


Harbors 

Historic Importance. — There has been a close relation 
between harbors, trade, and the spread of civilization ever 
since the Phoenicians carried their own alphabet and the 
products of the civilizations of both Egypt and Asia from 


586 


NEW PHYSIOGRAPHY 


Tyre and Sidon into Greece, Carthage, and the western 
Mediterranean world. Rome was the most convenient 
harbor on the borderland between Greek civilization and 
Etruscan civilization in Italy. After she had destroyed the 
harbor of her great rival, Carthage, Rome became the mis¬ 
tress of the Mediterranean world. 

During the Middle Ages and the Rennaissance, the pro¬ 
ducts of the civilizations of the East and West were dis¬ 
tributed largely by those cities that had good harbors, the 
Italian cities of Venice and Genoa, and the Hansa cities of 
Germany. 

In modern times Holland, Spain, England, the United 
States, and Germany have owed no small part of their 
rapid advance in power and wealth to their numerous good 
harbors. 

Definitions. — The terms haven, harbor, and port are each 
defined by the dictionaries as a sheltered recess in the shore 
line. It is true that each of the three requires safety for the 
vessels, but the harbor is more than a haven; it requires 
means for loading and unloading vessels and for landing 
passengers. 

A port is really a gateway or entrance and is defined in 
law as a place where persons and merchandise may legally 
enter or leave a country; i.e ., where customs officials are 
stationed to see that the laws of the nation concerning such 
entry or departure are obeyed. In this sense a port may be 
far from any body of water; it may be a place where a rail¬ 
road or a main wagon road passes from one country to 
another. 

The location of a harbor is necessarily limited to places 
where it is convenient to transfer passengers and merchan¬ 
dise from vessels to land conveyances, or from land con¬ 
veyances to vessels. 

What Makes A Good Harbor? — (1) A good harbor 
must be, first of all, a place of safety, which implies protec¬ 
tion from both winds and waves. (2) It must have enough 


SHORE LINES AND HARBORS 


587 


docks to handle the commerce. (3) The piers must be long 
enough for the longest vessels. (4) The water must be deep 
enough in the entrance and alongside the,docks to permit the 
largest vessels to enter and to dock. (5) At different docks 
there must be facilities for handling different kinds of mer¬ 
chandise; e.g., pumps for loading and unloading tankers, 
elevators for grain, clam-shell buckets for coal, and derricks 



Fig. 337. — The Drowned Valley of the Hudson 

The Palisades with their glaciated upper surface and their columnar structure 
are seen in the distance. (Photo by Brown Brothers.) 


for heavy articles, such as automobiles, in cases. (6) There 
must be sufficient anchorage ground, preferably without rock 
bottom, to meet the demands for such space. (7) The 
harbor should be ice-free in winter . This necessity was 
shown in the war between Russia and Japan when impor¬ 
tant vessels of the Russian Navy were icebound in the harbor 
of Vladivostock. (8) A rich hinterland and waterways lead¬ 
ing inland each increase the importance of a harbor; whereas 
mountains that interfere with travel between the hinterland 
and the harbor decrease its importance. 




588 


NEW PHYSIOGRAPHY 


New York Harbor. — “ The city of New York has become 
the metropolis of North America because of the natural ad¬ 
vantages of its location, rather than because of the acumen 
of its business men.” 

Let us see whether its advantages justify this statement. 

Figure 337 is a photograph, showing the drowned valley of 
the Hudson, one of the best anchorages in the harbor. The 
Palisades are seen across the river. 

Figure 338 is a map of New York Harbor, reduced from 
the large map by the United States Geological Survey. 
Numbers indicating in feet the elevation of the land sur¬ 
rounding the harbor are substituted for contour lines. 

Location. — New York is located at a corner in the shore 
line. The shores of Long Island have an easterly trend, as 
indicated by Coney Island and Rockaway Beach (Figure 
338); and the shore of New Jersey has a southerly trend as 
indicated by Sandy Hook (Figure 338), and by Figures 325 
and 329 of the Jersey shore. The harbor is surrounded by 
land on all points of the compass except between these 
easterly and southerly shore lines. 

Waves , therefore, can enter the harbor only when they come 
from the southeast and storms that come from this direction 
are rarely violent. 

The average harbor is situated on a straight-line coast. 
It is exposed to waves coming from any direction within an 
angle of 180°. Such is the case at San Francisco, Puget 
Sound, Chesapeake Bay, Rio Janeiro, and Buenos Ayres. 

The Upper New York Bay and the East and North Rivers 
are further protected from waves by the Narrows, only a 
mile wide. It is quite unusual for so large a harbor to be so 
well protected from waves. 

Protection From Winds. — Some years ago England, 
Germany, and the United States each sent two warships to 
the Samoan Islands. They maneuvered within the atoll for 
many weeks. One day an approaching hurricane was 
sighted. One of the American ships had steam enough for 


SHORE LINES AND HARBORS 


589 



Fig. 338 — New York Harbor 


























590 


NEW PHYSIOGRAPHY 


headway and sailed out of the harbor. Returning after the 
storm subsided, she found that all of the ships had been 
dashed upon the shore and shipwrecked, with serious losses. 
Atoll harbors are not protected from the wind, and this 
disaster illustrates the importance of such protection. 

The section of New York Harbor that is best protected 
from the wind is the estuary known as the Hudson River, 
and this is also the best anchorage. The Palisades extend 
along the west side of the Hudson River for about 30 miles, 
their height varying from 250 feet opposite Fifty-ninth Street 
to 500 feet at Hastings, and more than 800 feet at Haver- 
straw. This wall of rock effectually protects vessels in the 
river from winds coming from any point of the compass between 
southwest and due north. 

Northeast winds are sometimes violent, but this anchorage 
is also protected from them by the rocky shore shown on 
the right in Figure 338 which varies from 100 feet in height 
at Central Park to 500 feet at Croton River. 

The Upper Bay, with its 18 or 20 square miles of potential 
anchorage ground, has hills from 60 to 100 feet high near 
its western shore and is further protected by the Watchung 
Mountains with a height of 866 feet near Paterson. 
The most important docks are located along the shores 
of the Hudson and East Rivers and share the protection 
with the anchorages. 

The Lower Bay and Jamaica Bay are not without protec¬ 
tion from the winds. The terminal moraine that formed the 
Narrows may be traced from the southwest corner of Staten 
Island by the row of figures indicating heights from 100 feet 
to 380 feet on Staten Island and from 100 feet to 200 feet on 
Long Island. These bays may become valuable additions 
to New York Harbor as commerce increases. 

Connecting Waterways. — Perhaps the characteristic that 
did most to make New York the metropolis of the country 
was the waterways leading inland from the harbor, particu¬ 
larly the Hudson and its tributary, the Mohawk. These 


SHORE LINES AND HARBORS 


591 


streams lead to the lowest pass across the Appalachian ridges , 
only 442 feet above sea level at its highest point, whereas 
other passes are from three to four times as high. West of 
the Mohawk pass there is level land to the Great Lakes. 

Even before the Erie Canal was built, freight and passen¬ 
gers could be carried over this route more economically 
than over any other route between the west and the coast; 
and after the canal was finished the reduced expense of 
transportation brought so much freight to New York that 
ocean vessels could always find return cargoes there. Thus 
the commerce of the city increased faster than that of any of 
the rival cities, Boston, Philadelphia, and Baltimore. The 
Hudson-Champlain route to the St. Lawrence Basin was im¬ 
portant in the Revolution and earlier wars, and now it affords 
a valuable route for railroads and for motor cars. In the 
early days Long Island Sound, into which many New Eng¬ 
land rivers emptied, the Passaic, and several smaller streams 
were much more important than they are to-day and brought 
much business to the growing city. 

The hinterland of New York is more than half United 
States, and during the winter months it includes much of 
Canada as well. 

Other Advantages. — The harbor of New York is farther 
north than any other ice-free harbor of any large city on 
the Atlantic coast of North America. Boston, and Portland, 
Maine, have good harbors, but they are not ice-free. 

Disadvantages. — The entrance to New York Harbor is 
so crooked that vessels have to employ a pilot to guide them 
into it. A second disadvantage is the sediment brought into 
the harbor by rivers, which fills channels or builds bars in un¬ 
expected places. Much dredging is needed every year to 
keep the channels open. 

Submerged Valley Harbors. — The harbors of New York, 
Philadelphia, San Francisco, Seattle, Montreal, Quebec, 
Liverpool, Bristol, London, Shanghai, Hamburg, and scores 
more of the important harbors of the world belong to this 


592 


NEW PHYSIOGRAPHY 


class. The majority of the valleys submerged were formed 
by stream erosion. Even the great harbor of San Francisco, 
sometimes classed as a mountain-range harbor, is really 
the submerged valley of the Sacramento-San Joaquin river, 
which reaches the ocean through the “ Golden Gate ” that 
the river cut through the coast range. 

Advantages. — As a rule, these harbors are large, deep, and 
well protected. Many of them have connecting waterways 
that extend far inland. For example, the St. Lawrence con¬ 
nects the harbors of Quebec and Montreal with the Great Lakes 
and more than 1500 miles of navigable water. Hamburg 
receives freight from the Austrian frontier and Shanghai 
from far across China. The large volume of water that flows 
into and out of some of these harbors at each change of 
tide, keeps the entrance to the harbor open. 

Disadvantages. — Some drowned valley harbors have to 
contend with a shallow entrance, as at Liverpool, where a 
bar across the mouth of the Mersey prevents the passage of 
large vessels except at high tide. Some of them have a crooked 
entrance as was formerly the case with New York Harbor. 
The modern ocean liner, 1000 or more feet long, could not 
steer through the crooked entrance; so the old Ambrose 
Channel, which was only 16 feet deep at low tide, had to 
be widened, deepened, and straightened. 

This was done at a cost of about $6,000,000, and we now 
have a channel 2000 feet wide, 40 feet deep at low tide, and 
seven miles long, with but one curve. Some submerged valley 
harbors have excessively high tides. At Liverpool they formerly 
had considerable difficulty in loading and unloading vessels 
that were raised or lowered 20 feet between tides. 

The difficulty was partially overcome by building an 
immense floating raft or “ landing stage ” with drawbridge¬ 
like approaches similar to the hinged approaches that enable 
us to drive our motor cars on to ferry boats. 

Crater Harbors. — The harbor of Ischia in the Bay of 
Naples occupies the crater of the volcano of the same name, 


SHORE LINES AND HARBORS 


593 


whose last activity was described on page 561. Such 
harbors are usually well protected, but they frequently 
have a rocky bottom which causes the loss of many 
anchors. The island of St. Paul, near Ischia, also has a 
crater harbor, and so has the island of St. Thomas in the 
West Indies. The depression is due to the shrinkage of the 
cooling lava that formed the crater. 

Delta Harbors. — Among the important delta harbors 
are those at Para in the mouths of the Amazon, New Orleans, 
in the Mississippi, Calcutta in the Ganges, and in the Yellow 
River of China. They have the advantage of a large area 
of navigable water in the distributaries, and of long, connec¬ 
ting inland waterways, but are subject to two disadvantages: 
(1) The shifting of the principal mouth and (2) the forma¬ 
tion of bars at the mouths of the distributaries. The latter 
was overcome in the Mississippi delta by building jetties, 
which narrowed the channel, causing the river to scour out 
the sediment between them and to carry the river’s load out 
into deeper water. In time a bar across the entrance will 
be formed in this deeper water, and then the jetties will 
have to be lengthened again. 

Fiord Harbors. — The water in fiords is usually very deep 
close to shore, because of the Z7-shaped cross-section. This 
enables ships to lie close to shore. These harbors are well 
protected and often have a straight, wide and deep entrance 
(Figure 335). 

Their high steep sides tend to make loading and unload¬ 
ing vessels difficult and to make the harbor inaccessable. 
Mountains sometimes surround fiords, increasing the diffi¬ 
culty of access. Few fiord harbors are important. 

This is partly because of the disadvantages mentioned 
and partly because the demands of commerce are not great 
in these high latitudes. Perhaps the fiord harbor at Oslo, 
Norway, is the most important of the kind. 

Lagoon Harbors. — Some lagoon harbors are located 
behind barrier beaches, others behind sand spits, and still 


594 


NEW PHYSIOGRAPHY 


others behind coral reefs. In each case the harbor is well pro¬ 
tected from all ordinary storm waves, but not from landward 
storm winds. The entrance to lagoon harbors is sometimes 
shallow, narrow, crooked, and dangerous, particularly when 
the barrier is a coral reef. Harbors behind sand reefs are 
usually very shallow and, if the area of the harbor is large, 
may have strong tidal currents through the inlet. The inlets 
to harbors behind such barriers are often being filled, and 
others are being opened. It is believed that there was an 
inlet through Sandy Hook near Atlantic Highlands in revo¬ 
lutionary times. 1 

Barnegat Bay (Figure 329) is a large harbor behind a sand 
spit, with an entrance only seven or eight feet deep at 
midtide. 

Jamaica Bay (Figure 338) is another sand-spit harbor as 
is also that at Erie, Pennsylvania. The harbor at Atlantic 
City, New Jersey is enclosed by offshore bars. 

The bay of Biscayne, Florida, is an example of a coral 
barrier reef harbor. That of Hamilton, Bermuda, is an 
atoll harbor. 

Pearl Harbor, Hawaii, and the island of Guam are the most 
important coral-reef harbors belonging to the United States. 

Island Harbors. -— Many harbors are protected from wind 
and wave by islands that separate them from the sea. The 
harbor of Vancouver is behind a large island that protects 
it from the prevailing northwest wind. 

That of Callao, Peru, is behind an offshore island. The 
latitude of Callao is 15° south, and therefore the region is 
subject to the southeast trades most of the year and to the 
northwest hooked trades for the remaining months. 

Many of the piers of Boston Harbor lie along the sub¬ 
merged valleys of the Charles and the Mystic Rivers, but 
a larger area is protected by an island of glacial origin. 

Artificial Harbors. — Harbors have been built at both 
ends of the Panama Canal and on the Pacific side of the 

described in Cooper’s Water Witch. 


SHORE LINES AND HARBORS 595 

Isthmus of Tehuantepec. A well-protected harbor has been 
devised at La Plata near Buenos Ayres by building great 
breakwaters in the open roadstead of the La Plata River. 
The naval harbor at Dover, England, is one of the best 
artificial harbors in the world. Here great concrete break¬ 
waters enclose a deep area of about a square mile. The 
harbor of Hilo, Hawaii, has a breakwater two miles long; 
and one of about the.same length.at San Pedro, California, 


Fig. 339. — The Mousehole, the Little Toy Harbor on the Coast of Cornwall 



makes a harbor for Los Angeles. Figure 339 shows the 
“ Mousehole/’ a “ toy harbor ” on the coast of Cornwall, 
with a masonry breakwater. 

Destruction of Harbors. — Rivers silt up shallow coastal 
plain harbors, and currents along shore close harbor en¬ 
trances. Mangrove and other forms of plant life, and corals 
and other forms of animal life, fill or obstruct them. 

The destruction of the harbors of an enemy is one of the 
most effective war measures. Alexander the Great destroyed 
the harbor of Tyre; the Romans that of Carthage; and the 
Dutch long kept the entrance of Antwerp closed. During 






596 


NEW PHYSIOGRAPHY 


the Napoleonic wars, England endeavored to blockade the 
ports of France and all her allies. 

In recent years blockades have been equally effective as 
war measures. The American blockades of Manila and 
Santiago, in 1898, materially shortened the Spanish-American 
War, and the allied blockade of the ports of the Central 
Powers, from 1914 to 1918, prevented the blockaded countries 
from getting many needed supplies, such as oils, fats, and 
fertilizers; and also made it possible for the American army 
to cross the ocean, thus making the blockade one of the most 
important measured of the war. 

QUESTIONS 

1. Give reasons for believing that the shore lines migrate and 
state what may cause this migration. 

2. Compare in a table the effects of (1) elevation, (2) depression 
and (3) the action of waves, currents, and tides at- shore line. 

3. Account for branched appearance of Chesapeake Bay, the 
even shore line of Peru, the deltas of the Mediterranean Sea, the 
tide ascending the Hudson to Albany. 

4. By what means can one locate former shore lines of extinct 
lakes, such as Bonneville and Passaic? 

















































































































































































































































